Avoid race while processing secondary transaction events.

git-svn-id: http://yate.null.ro/svn/yate/trunk@5679 acf43c95-373e-0410-b603-e72c3f656dc1
Safely reset media in hangup.

git-svn-id: http://yate.null.ro/svn/yate/trunk@5680 acf43c95-373e-0410-b603-e72c3f656dc1