res_pjsip_nat: Don't assume a Contact header will always contain a URI.

If the 'rewrite_contact' option was enabled and a Contact header was received
which contained a '*' a crash would occur.

This change makes the res_pjsip_nat module ignore the Contact header if it
contains only a '*'.

(closes issue ASTERISK-23101)
Reported by: Matt Jordan
........

Merged revisions 405019 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@405020 65c4cc65-6c06-0410-ace0-fbb531ad65f3