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
........

Merged revisions 390044 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@390047 f38db490-d61c-443f-a65b-d21fe96a405b