mds/Server.cc: fix possible NULL pointer dereference

CID 1019558 (#1 of 1): Dereference after null check (FORWARD_NULL)
  var_deref_model: Passing null pointer "straydn" to function
  "CDentry::get_dir() const", which dereferences it.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>