Use PJ_ASSERT_RETURN() on pjsip_auth_create_digest() and pjsua_init_tpselector() (#3009)

* Use PJ_ASSERT_RETURN on pjsip_auth_create_digest

* Use PJ_ASSERT_RETURN on pjsua_init_tpselector()

* Fix incorrect check.

* Add return value to pjsip_auth_create_digest() and pjsip_auth_create_digestSHA256()

* Modification based on comments.