mds/MDCache.cc: fix dereference after null check

CID 716992 (#1 of 1): Dereference null return value (NULL_RETURNS)
  dereference: Dereferencing a pointer that might be null "dir"
  when calling "MDSCacheObject::is_auth() const".

Add assert for 'dir' before call assert on dir->is_auth().

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