svn_repos__post_commit_error_str() edge case segfault

Found this while reviewing the 1.6.x-svn_fs_commit_txn backport, but
it's present in trunk as well:

When ERR does not contain SVN_ERR_REPOS_POST_COMMIT_HOOK_FAILED,
svn_repos__post_commit_error_str() sets hook_err2 to NULL (in the else{}
on line 629) and later dereferences it (to get the ->message member).

The caller in libsvn_repos/commit.c passes svn_repos_fs_commit_txn()'s
return value directly to svn_repos__post_commit_error_str(), so the ERR
parameter of svn_repos__post_commit_error_str() may in fact not contain
a SVN_ERR_REPOS_POST_COMMIT_HOOK_FAILED link.
Received on 2011-04-22 00:45:00 CEST