Fix crash when AMI redirect action redirects two channels out of a bridge.

The two party bridging loops were changing the bridge peer pointers
without the channel locks held.  Thus when ast_channel_massquerade()
tested and used the pointer there is a small window of opportunity for the
pointers to become NULL even though the masquerade code has the channels
locked.

(closes issue ASTERISK-21356)
Reported by: William luke
Patches:
      jira_asterisk_21356_v11.patch (license #5621) patch uploaded by rmudgett
Tested by: William luke
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Merged revisions 386256 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 386286 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@386289 65c4cc65-6c06-0410-ace0-fbb531ad65f3