chan_sip: Fix reference leaks in error paths.

* get_sip_pvt_from_replaces leaks sip_pvt_ptr on any error.
* build_peer leaks peer on failure to allocate the endpoint.

This patch fixes get_sip_pvt by using an RAII_VAR, build_peer is fixed
with an unref in the appropriate place.

ASTERISK-26184 #close

Change-Id: I728b424648ad041409f7d90880f4c28b3ce2ca12