Prevent crash when an SDP offer is received with an encrypted video stream when support for video is disabled and res_srtp is loaded.

(closes issue ASTERISK-19202)
Reported by: Catalin Sanda
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Merged revisions 351504 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 351505 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@351506 65c4cc65-6c06-0410-ace0-fbb531ad65f3