Fix a potential error leak.

* subversion/libsvn_fs/fs-loader.c
  (svn_fs_lock, svn_fs_unlock): Handle errors in a manner similar to
   the repos functions.

Found by: julianfoad


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1582845 13f79535-47bb-0310-9956-ffa450edef68