res_pjsip_endpoint_identifier_ip.c: Fix apply identify validation.

The ip_identify_apply() did not validate the configuration for simple
static configuration errors or deal well with address resolution errors.

* Added missing configuration validation checks.
* Fixed address resolution error handling.
* Demoted an error message to a warning since it does not fail applying
the identify object configuration.

Change-Id: I8b519607263fe88e8ce964f526a45359fd362b6e