mds/Migrator.cc: fix possible dereference NULL return value

CID 716997 (#1 of 1): Dereference null return value (NULL_RETURNS)
  dereference: Dereferencing a pointer that might be null "in" when
  calling "MDSCacheObject::is_auth() const".

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