fix iax_frame double free

fix iax_frame double free

Very unfortunate things happen if we add an iax_frame
to the frame queue and let go of the lock before scheduling
the frame's transmit... There is a race condition that
exists where the frame can be removed from the frame_queue
and freed before the transmit is scheduled if we do not
hold on to that lock.  This results in a freed frame
being scheduled for transmit later.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@263151 65c4cc65-6c06-0410-ace0-fbb531ad65f3