rtpdec: Read the packet length for all RTCP packet types

This allows skipping past unsupported RTCP packet types, as
RFC 3550 section 6.1 mandates.

Currently this only has any practical effect if a sender puts
an unrecognized type before RTCP_BYE in a compounded packet, or
(incorrectly) does not put RTCP_SR first.

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