Fix stuck DTMF when bridge is broken.

Fix stuck DTMF when bridge is broken.

When a bridge is broken by an AMI Redirect action or the ChannelRedirect
application, an in progress DTMF digit could be stuck sending forever.

* Made simulate a DTMF end event when a bridge is broken and a DTMF digit
was in progress.

(closes issue ASTERISK-20492)
Reported by: Jeremiah Gowdy
Patches:
      bridge_end_dtmf-v3.patch.txt (license #6358) patch uploaded by Jeremiah Gowdy
      Modified to jira_asterisk_20492_v1.8.patch
      jira_asterisk_20492_v1.8.patch (license #5621) patch uploaded by rmudgett
Tested by: rmudgett

Review: https://reviewboard.asterisk.org/r/2169/
........

Merged revisions 375964 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@375965 65c4cc65-6c06-0410-ace0-fbb531ad65f3