res_pjsip_session.c: Restructure ast_sip_session_alloc()

* Restructure ast_sip_session_alloc() to need less cleanup on off nominal
error paths.

* Made ast_sip_session_alloc() and ast_sip_session_create_outgoing() avoid
unnecessary ref manipulation to return a session.  This is faster than
calling a function.  That function may do logging of the ref changes with
REF_DEBUG enabled.

Change-Id: I2a0affc4be51013d3f0485782c96b8fee3ddb00a