fixes issue with double "sip:" in header field

This is a clear mistake in logic.  Future discussions
about how to avoid having to handle uri's like this
should take place in the future, but this fix needs
to go in for now.

(closes issue #15847)
Reported by: ebroad
Patches:
      doublesip.patch uploaded by ebroad (license 878)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@258305 65c4cc65-6c06-0410-ace0-fbb531ad65f3