mds/MDS.cc: fix dereference null return value

Fix for:

returned_null: Function "SessionMap::get_session(entity_name_t)" returns
  null (checked 12 out of 14 times)

CID 739601 (#1 of 1): Dereference null return value (NULL_RETURNS)
  dereference: Dereferencing a pointer that might be null
  "this->sessionmap.get_session(entity_name_t::CLIENT(client.v))" when
  calling "MDS::send_message_client_counted(Message *, Session *)"

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