Correct the previous nil-guard: dropping the answer prevents the crash but
breaks subscribe renegotiation. In single-PC (UseSinglePeerConnection) and
one-shot signalling modes the single publisher PC carries both directions;
when the server renegotiates to push a subscribed track it offers on that PC
(NegotiateSubscriber -> publisher.Negotiate -> signalSendOffer) and the client
replies with an answer. Route that answer to the publisher, matching every
other subscriber-path method in TransportManager (GetSubscriberRTT,
AddTrackLocal, AddSubscribedTrack, NegotiateSubscriber, ...).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
