Simplify reporting of API violations of the conflict resolver callback.

* subversion/libsvn_wc/merge.c
  (svn_wc__merge_internal): Replace error-checking code and custom error
    messages by SVN_ERR_ASSERT statements. (3 instances.)
* subversion/libsvn_wc/props.c
  (maybe_generate_propconflict): Same. (2 instances.) Also don't bother
    setting an output parameter when we're returning an error, as callers
    should never look at it.

Index: subversion/libsvn_wc/merge.c
===================================================================
--- subversion/libsvn_wc/merge.c	(revision 32020)
+++ subversion/libsvn_wc/merge.c	(working copy)
@@ -416,10 +416,7 @@ svn_wc__merge_internal(svn_stringbuf_t *
               cdesc.property_name = NULL;
 
               SVN_ERR(conflict_func(&result, &cdesc, conflict_baton, pool));
-              if (result == NULL)
-                return svn_error_create(SVN_ERR_WC_CONFLICT_RESOLVER_FAILURE,
-                                        NULL, _("Conflict callback violated API:"
-                                                " returned no results"));
+              SVN_ERR_ASSERT(result);
 
               if (result->save_merged)
                 {
@@ -732,10 +729,7 @@ svn_wc__merge_internal(svn_stringbuf_t *
           cdesc.property_name = NULL;
 
           SVN_ERR(conflict_func(&result, &cdesc, conflict_baton, pool));
-          if (result == NULL)
-            return svn_error_create(SVN_ERR_WC_CONFLICT_RESOLVER_FAILURE,
-                                    NULL, _("Conflict callback violated API:"
-                                            " returned no results"));
+          SVN_ERR_ASSERT(result);
 
           switch (result->choice)
             {
@@ -775,26 +769,16 @@ svn_wc__merge_internal(svn_stringbuf_t *
                 }
               case svn_wc_conflict_choose_merged:
                 {
-                  if (! result->merged_file)
-                    {
-                      /* Callback asked us to choose its own
-                         merged file, but didn't provide one! */
-                      return svn_error_create
-                          (SVN_ERR_WC_CONFLICT_RESOLVER_FAILURE,
-                           NULL, _("Conflict callback violated API:"
-                                   " returned no merged file"));
-                    }
-                  else
-                    {
-                      SVN_ERR(svn_wc__loggy_copy
-                              (log_accum, NULL, adm_access,
-                               svn_wc__copy_translate,
-                               result->merged_file, merge_target,
-                               FALSE, pool));
-                      *merge_outcome = svn_wc_merge_merged;
-                      contains_conflicts = FALSE;
-                      goto merge_complete;
-                    }
+                  SVN_ERR_ASSERT(result->merged_file);
+
+                  SVN_ERR(svn_wc__loggy_copy
+                          (log_accum, NULL, adm_access,
+                           svn_wc__copy_translate,
+                           result->merged_file, merge_target,
+                           FALSE, pool));
+                  *merge_outcome = svn_wc_merge_merged;
+                  contains_conflicts = FALSE;
+                  goto merge_complete;
                 }
               case svn_wc_conflict_choose_postpone:
               default:
Index: subversion/libsvn_wc/props.c
===================================================================
--- subversion/libsvn_wc/props.c	(revision 32020)
+++ subversion/libsvn_wc/props.c	(working copy)
@@ -20,7 +20,6 @@
 
 #include <stdlib.h>
 #include <string.h>
-#include <assert.h>
 #include <apr_pools.h>
 #include <apr_hash.h>
 #include <apr_tables.h>
@@ -1387,13 +1386,7 @@ maybe_generate_propconflict(svn_boolean_
 
   /* Invoke the interactive conflict callback. */
   SVN_ERR(conflict_func(&result, cdesc, conflict_baton, pool));
-  if (result == NULL)
-    {
-      *conflict_remains = TRUE;
-      return svn_error_create(SVN_ERR_WC_CONFLICT_RESOLVER_FAILURE,
-                              NULL, _("Conflict callback violated API:"
-                                      " returned no results."));
-    }
+  SVN_ERR_ASSERT(result);
 
   switch (result->choice)
     {
@@ -1428,26 +1421,19 @@ maybe_generate_propconflict(svn_boolean_
         }
       case svn_wc_conflict_choose_merged:
         {
-          if (!cdesc->merged_file && !result->merged_file)
-            return svn_error_create
-                (SVN_ERR_WC_CONFLICT_RESOLVER_FAILURE,
-                 NULL, _("Conflict callback violated API:"
-                         " returned no merged file."));
-          else
-            {
-              svn_stringbuf_t *merged_stringbuf;
-              svn_string_t *merged_string;
+          svn_stringbuf_t *merged_stringbuf;
+          svn_string_t *merged_string;
 
-              SVN_ERR(svn_stringbuf_from_file2(&merged_stringbuf,
-                                               result->merged_file ?
-                                                    result->merged_file :
-                                                    cdesc->merged_file,
-                                               pool));
-              merged_string = svn_string_create_from_buf(merged_stringbuf, pool);
-              apr_hash_set(working_props, propname,
-                           APR_HASH_KEY_STRING, merged_string);
-              *conflict_remains = FALSE;
-            }
+          SVN_ERR_ASSERT(cdesc->merged_file || result->merged_file);
+          SVN_ERR(svn_stringbuf_from_file2(&merged_stringbuf,
+                                           result->merged_file ?
+                                                result->merged_file :
+                                                cdesc->merged_file,
+                                           pool));
+          merged_string = svn_string_create_from_buf(merged_stringbuf, pool);
+          apr_hash_set(working_props, propname,
+                       APR_HASH_KEY_STRING, merged_string);
+          *conflict_remains = FALSE;
           break;
         }
     }