Fix issue with a T38 reinvite being sent even if not configured to do so.

If we receive a T38 request negotiate control frame we should only attempt to do so
if the option is enabled on the dialog.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@184280 65c4cc65-6c06-0410-ace0-fbb531ad65f3