conncheck: Don't consider unmatched STUN response as media

Previously, a STUN response for which there was no associated request would be considered valid media input and as such could keep a dead connection alive. If peer A was communicating with peer B and peer B got disconnected, the keepalive mechanism in peer A should detect this. However, a misbehaving STUN server could keep sending STUN responses to peer A which would then be considered a valid communication between A and B and thereby prevent the keepalive mechanism from shutting down the connection.