mds/Server.cc: check straydn before dereference

ID 1019554 (#1 of 1): Dereference after null check (FORWARD_NULL)
 13. var_deref_model: Passing null pointer "straydn" to function
 "MDSCacheObject::is_auth() const", which dereferences it.

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