Truncate CSRC list in RtpPacket::SetCsrcs if too large

Truncates the CSRC list to a maximum of 15 entries as per the 4-bit RTP
header field limitation, and logs a warning when truncation occurs. Also
updates the buffer capacity check and allocation to happen prior to
writing data to avoid heap buffer overflow.

Additionally, ensure that the underlying buffer size is updated before
writing the CSRC data. By calling SetSize prior to WriteAt, we prevent
potential heap buffer overflows that could occur if the buffer was not
sufficiently allocated before the write operation.

Bug: chromium:486317116
Change-Id: I2821867f94c0a8d174fb25047ac1f4b94ee5b867
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/459943
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47266}