Bridging: Fix a behavioral change when checking if a channel is leaving a bridge

r420934 introduced some failures in the test suite.  Upon investigating, it was
discovered that differences in the way we were evaluating whether a channel was in
the process of leaving a bridge were causing some reinvites not to occur (mostly
reinvites back to Asterisk when ending a call). This patch fixes that behavioral
change.

ASTERISK-24027 #close
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/3910/
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Merged revisions 421186 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 421187 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@421195 65c4cc65-6c06-0410-ace0-fbb531ad65f3