Protect a piece of code against future coding errors.

* subversion/libsvn_fs_fs/fs_fs.c
  (get_revprop_packname): Prevent a stringbuf whose invariants no longer hold
    from being used.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1515329 13f79535-47bb-0310-9956-ffa450edef68