[PATCH] Fix for access violation in svn_fs__path_valid()

Hi,

as discussed on the dev list (topic: inconsistent null-ptr handling in
utf-path related functions):
http://mail-archives.apache.org/mod_mbox/subversion-dev/201601.mbox/browser
attached is a patch to prevent svn_fs__path_valid() to run in an access
violation when creating an error due to path being invalid (in this case
NULL).

[[
Prevent an access violation upon an invalid call, while generating an error
message.

* libsvn_fs/fs-loader.c:
    (svn_fs__path_valid): add null-check for path-parameter when
generating the
                           error and use "NULL" in the message, in case
                           it actually is null.
]]

Regards,
Stefan