Fix a segfault with svnserve and pre-1.5 FSFS repositories.

* subversion/libsvn_repos/config_pool.c
  (find_repos_config): Don't try to map checksums that we don't know.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1547455 13f79535-47bb-0310-9956-ffa450edef68