Add a separate buffer for SRTCP packets

The function ast_srtp_protect used a common buffer for both SRTP and SRTCP
packets. Since this function can be called from multiple threads for the same
SRTP session (scheduler for SRTCP and channel for SRTP) it was possible for the
packets to become corrupted as the buffer was used by both threads
simultaneously.

This patch adds a separate buffer for SRTCP packets to avoid the problem.

(closes issue ASTERISK-18889, Reported/patch by Daniel Collins)
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Merged revisions 347995 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 347996 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@348567 65c4cc65-6c06-0410-ace0-fbb531ad65f3