Fix segfault when dealing with chan_agent channels.

Check the returned bridged pointer for NULL to avoid a crash.  It looks
like chan_agent is returning a NULL pointer when it probably should be
returning a pointer to the channel the Agent channel is pretending to be.

(closes issue ASTERISK-21793)
Reported by: Rodrigo P. Telles
Patches:
      jira_asterisk_21793_v1.8.patch (license #5621) patch uploaded by rmudgett
Tested by: Rodrigo P. Telles
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Merged revisions 390044 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 390047 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@390068 65c4cc65-6c06-0410-ace0-fbb531ad65f3