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>