rtpdec: Always check if we have the next packet queued

It doesn't matter what the actual reason for not returning
an AVPacket was - if we didn't return any packet and we have
the next one queued, parse it immediately. (rtp_parse_queued_packet
always consumes a queued packet if one exists, so there's no risk
for infinite loops.)

Signed-off-by: Martin Storsjö <martin@martin.st>
