rtc: guard against nil subscriber in TransportManager.HandleAnswer (#4680)

In single-PC and one-shot signalling modes the subscriber PCTransport is
never created, so t.subscriber is nil. Every other TransportManager method
that touches the subscriber nil-checks it first; HandleAnswer did not, so a
client sending an SDP answer in those modes crashed the process with a nil
pointer dereference. Guard it and log, matching AddICECandidate/Negotiate.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

pkg/rtc/transportmanager.go

@@ -560,6 +560,11 @@ func (t *TransportManager) ProcessPendingPublisherOffer() {
 }
 
 func (t *TransportManager) HandleAnswer(answer webrtc.SessionDescription, answerId uint32) {
+	if t.subscriber == nil {
+		// no subscriber transport in single-PC / one-shot signalling mode
+		t.params.Logger.Warnw("answer received, but no subscriber peer connection", nil)
+		return
+	}
 	t.subscriber.HandleRemoteDescription(answer, answerId)
 }