Correct buffer handling for RTCP packets

Previous code could read 4 bytes past the end of the buffer on a RTCP_SR
packet or offset a pointer by an unchecked external value (payload_len),
though neither will reliably cause a crash or other misbehavior beyond
garbage timestamps.

Additionally, unknown RTCP packet types, even in compounded packets, are
now ignored as per RFC 3550 section 6.1, page 22, though 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: John Brooks <john.brooks@bluecherry.net>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
