res_pjsip: When handling 401/407 responses don't assume a request will have an endpoint.

This change removes the assumption that an outgoing request will always
have an endpoint and makes the authenticate_qualify option work once again.

(closes issue ASTERISK-23210)
Reported by: Joshua Colp


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@410306 f38db490-d61c-443f-a65b-d21fe96a405b