Fix a segfault with svnserve and pre-1.5 FSFS repositories.
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: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1547455 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion