If the config file does not exist, then the first use crashes Asterisk.

(closes issue #13848)
 Reported by: klaus3000
 Patches: 
       udptl.c.patch uploaded by eliel (license 64)
 Tested by: blitzrage


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@159475 65c4cc65-6c06-0410-ace0-fbb531ad65f3