Ensure chan_sip rejects encrypted streams without crypto info

Ensure chan_sip rejects encrypted streams without crypto info

This ensures that Asterisk rejects encrypted media streams (RTP/SAVP
audio and video) that are missing cryptographic keys and ensures that
the incoming SDP is consistent with RFC4568 as far as having a crypto
attribute present for any SAVP streams.

Review: https://reviewboard.asterisk.org/r/2204/
........

Merged revisions 378217 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@378218 65c4cc65-6c06-0410-ace0-fbb531ad65f3