chan_mgcp: Properly handle malformed media lines

This corrects a situation in which a media line was not parsed properly
and resulted in a crash.

(closes issue ASTERISK-21190)
Reported by: adomjan
Patches:
    chan_mgcp.c-sscnaf_fix uploaded by adomjan (License 5448)
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Merged revisions 401537 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 401538 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 401539 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@401540 65c4cc65-6c06-0410-ace0-fbb531ad65f3