open_path(): silence compiler warning.

subversion/libsvn_fs_fs/tree.c: In function 'open_path':
subversion/libsvn_fs_fs/tree.c:916: warning: 'directory' may be used uninitialized in this function

* subversion/libsvn_fs_fs/tree.c
  (open_path):
    Set directory to NULL to silence a compiler warning.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1469520 13f79535-47bb-0310-9956-ffa450edef68