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/
........

Merged revisions 421186 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@421187 f38db490-d61c-443f-a65b-d21fe96a405b