chan_sip.c: Prevent address change on unauthenticated SIP request.

If the name of a peer is known and a SIP request is sent using that
peer's name, the address of the peer will change even if the request
fails the authentication challenge. This means that an endpoint can
be altered and even rendered unusuable, even if it was in a working
state previously. This can only occur when the nat option is set to the
default, or auto_force_rport.

This change checks the result of authentication first to ensure it is
successful before setting the address and the nat option.

ASTERISK-28589 #close

Change-Id: I581c5ed1da60ca89f590bd70872de2b660de02df