fixes ability to exit echo app

when called from a ISDN channel, null frames prevent '#' exit.
Now only echo back VOICE and DTMF frames

(issue #16880)
Reported by: alecdavis
Patches: 
      echo_exit.diff.txt uploaded by alecdavis (license 585)
Tested by: alecdavis



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@249801 65c4cc65-6c06-0410-ace0-fbb531ad65f3