Prevent crash when an SDP offer is received with an encrypted video stream...

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