In `srtp_unprotect_rtcp()` we are not validating that the packet
length is as long as the minimum required.  This would cause
`enc_octet_len` to underflow, which would cause us to try to decrypt
data past the end of the packet in memory -- a buffer over-read and
buffer overflow.

In `srtp_protect_rtcp()`, we were similarly not validating the packet
length.  Here we were also polluting the address of the SRTCP
encrypted flag and index (the `trailer`), causing us to write one word
to a bogus memory address before getting to the encryption where we
would also overflow.

In this commit we add checks to appropriately validate the RTCP/SRTCP
packet lengths.

`srtp_unprotect_rtcp_aead()` (but not protect) did correctly validate
the packet length; this check would now be redundant as the check in
`srtcp_unprotect_rtcp()` will also run first, so it has been removed.