fixes issue with channels not going down after transfer

Iax2 currently does not support native bridging if the timeoutms value is set.  We check for that in iax2_bridge, but then set timeoutms to 0 by default.  If the timeoutms is not provided it is set to -1. By setting timeoutms to 0 it is processed causing a bridging retry loop.

(closes issue #15216)
Reported by: oxymoron
Tested by: dvossel


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@198824 65c4cc65-6c06-0410-ace0-fbb531ad65f3