Don't leave *conflict_p uninitialized if svn_repos_fs_commit_txn() fails before calling the FS layer.
This function gives the same promises as svn_fs_commit_txn(), which always sets the conflict variable to either NULL, or to a valid conflict string. Make sure that we fulfil this contract even if the commit preparation fails — e.g., when calling pre-commit hook or removing ephemeral txnprops.
* subversion/libsvn_repos/fs-wrap.c (svn_repos_fs_commit_txn): Start from setting *conflict_p to NULL.
* subversion/tests/libsvn_repos/repos-test.c (commit_aborted_txn, test_funcs): Add a test for this behavior.