Some sip notify events don't have a content type, such as notifies that only…

Some sip notify events don't have a content type, such as notifies that only update the caller id info of a call.

src/mod/endpoints/mod_sofia/sofia.c

@@ -469,6 +469,7 @@ void sofia_handle_sip_i_notify(switch_core_session_t *session, int status,
 	if (switch_event_create(&s_event, SWITCH_EVENT_NOTIFY_IN) == SWITCH_STATUS_SUCCESS) {
 		switch_event_add_header_string(s_event, SWITCH_STACK_BOTTOM, "event", sip->sip_event->o_type);
 		switch_event_add_header_string(s_event, SWITCH_STACK_BOTTOM, "pl_data", sip->sip_payload ? sip->sip_payload->pl_data : "");
+		if ( sip->sip_content_type != NULL )
 			switch_event_add_header_string(s_event, SWITCH_STACK_BOTTOM, "sip_content_type", sip->sip_content_type->c_type);
 		switch_event_add_header(s_event, SWITCH_STACK_BOTTOM, "port", "%d", sofia_private->gateway->profile->sip_port);
 		switch_event_add_header_string(s_event, SWITCH_STACK_BOTTOM, "module_name", "mod_sofia");