bridge_native_rtp: Fix native bridge tech being incompatible when it should be.

bridge_native_rtp: Fix native bridge tech being incompatible when it should be.

When checking compatability for the native RTP bridge technology there is a
race condition between clearing framehooks that are destroyed when leaving
certain bridges with certain technologies (such as bridge_native_rtp) and
joining bridges with the bridge_native_rtp technology. Yes, that means a
channel in a native RTP bridge could move to another native RTP bridge and
be considered incompatible with the new native RTP bridge causing it to
revert to a simple bridge technology0. This fixes that bug by ignoring
framehooks that have been marked for destruction when checking for
compatibility with the bridge_native_rtp technology.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@391453 65c4cc65-6c06-0410-ace0-fbb531ad65f3