Avoid useless NAT log message

If FS is not behind NAT, then every call generates at least three
INFO-level log messages:

  [INFO] switch_nat.c:589 NAT port mapping disabled

This is useless noise.  The message is only interesting if you do have
NAT enabled but mapping disabled, which might indicate a configuration
issue.

With this change, we just skip the entire nat_add_mapping function if
the NAT system isn't initialized or we're not behind NAT.

src/switch_nat.c

@@ -585,6 +585,7 @@ SWITCH_DECLARE(switch_status_t) switch_nat_add_mapping_internal(switch_port_t po
 	switch_status_t status = SWITCH_STATUS_FALSE;
 	switch_event_t *event = NULL;
 
+	if (!initialized || !nat_globals.nat_type) return status;
 	if (!nat_globals.mapping) {
 		switch_log_printf(SWITCH_CHANNEL_LOG, SWITCH_LOG_INFO, "NAT port mapping disabled\n");
 		return status;