Since the packet entries are already overdimensioned, there's no
need to add additional padding.

In principle, we should be receiving receiver reports only, but let's
be robust.

This avoids locking in sendRR.

This fixes leaking of rtpUpSender goroutines.

This may happen if we close a connection before getting any tracks.

It would be better to terminate FIR, note that a keyframe was requested,
and send FIR on our own.

Thanks to Giuseppe Castagna for noticing.