Prevent possible race condition on dual redirect of channels in the same bridge.

Prevent possible race condition on dual redirect of channels in the same bridge.

The AST_FLAG_BRIDGE_DUAL_REDIRECT_WAIT flag was created to prevent bridges from
prematurely acting on orphaned channels in bridges. The problem with the AMI
redirect action was that it was setting this flag on channels based on the presence
of a PBX, not whether the channel was in a bridge. Whether a channel has a PBX
is irrelevant, so the condition has been altered to check if the channel is in a
bridge.

ASTERISK-24536 #close
Reported by Niklas Larsson

Review: https://reviewboard.asterisk.org/r/4268

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@429741 65c4cc65-6c06-0410-ace0-fbb531ad65f3