fixes infinite loop during udptl.c's decode_open_type

fixes infinite loop during udptl.c's decode_open_type

When decode_length returns the length there is a check to see if that
length is negative, if so the decode loop breaks as this means the
limit has been reached.  The problem here is that length is an
unsigned int, so length can never be negative.  This resulted in
an infinite loop.

(issue #17352)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@264400 65c4cc65-6c06-0410-ace0-fbb531ad65f3