res_pjsip.c: Fix endpoint identifier registration name search.

If an endpoint identifier name in the endpoint_identifier_order list is a
prefix to the identifier we are registering, we could install it in the
wrong position of the list.

Assuming
endpoint_identifier_order=username,ip,anonymous

then registering the "ip_only" identifier would put the identifier in the
wrong position of the priority list.

* Fix incorrect strncmp() string prefix matching.

Change-Id: Ib8819ec4b811da8a27419fd93528c54d34f01484