srtp: 1.5.2 -> 1.5.4 for CVE-2015-6360

Prevent potential DoS attack due to lack of bounds
checking on RTP header CSRC count and extension header
length. Credit goes to Randell Jesup and the Firefox team
for reporting this issue.

https://www.rapid7.com/db/vulnerabilities/freebsd-vid-6171eb07-d8a9-11e5-b2bd-002590263bf5