res_pjsip_transport_websocket: Fix crash when the Contact header is not a URI.

The code for changing the Contact header wrongly assumed that the Contact
would always contain a URI. This is incorrect.

ASTERISK-24271
Reported by: Dafi Ni


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@422557 f38db490-d61c-443f-a65b-d21fe96a405b