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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@405019 f38db490-d61c-443f-a65b-d21fe96a405b