res_rtp_multicast: Ensure SSRC is set properly

This fixes a bug where the SSRC field on multicast RTP can be stuck at
0 which can cause problems for endpoints trying to make sense of
incoming streams.

(closes issue ASTERISK-22567)
Reported by: Simone Camporeale
Patches:
    22567_res_mulitcast_ssrc.patch uploaded by Simone Camporeale (License 6536)
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Merged revisions 400393 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 400394 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 400395 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400397 65c4cc65-6c06-0410-ace0-fbb531ad65f3