Ensure res_rtp_mutlicast sets SSRC 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)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@400393 f38db490-d61c-443f-a65b-d21fe96a405b