Fixes segfault when switching UDP to TCP in sip.conf after reload.

If transport in sip.conf is switched from UDP to TCP, Asterisk segfaults right after issuing a sip reload.  The problem is the socket type is changed to TCP but the fd may still be present for UDP.  Later, when the TCP session should be created or set using an existing one, it isn't because the old file descriptor is still present.  Now every time transport is changed during a sip.conf reload, the file descriptor is set to -1, signifying it must be created or found.

(closes issue #14727)
Reported by: pj
Tested by: dvossel

Review: http://reviewboard.digium.com/r/229/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@189771 65c4cc65-6c06-0410-ace0-fbb531ad65f3