fix null warning

src/mod/endpoints/mod_sofia/sofia.c

@@ -8283,7 +8283,9 @@ void sofia_handle_sip_i_invite(nua_t *nua, sofia_profile_t *profile, nua_handle_
 					}
 				}
 
+				if (is_nat) {
 					switch_log_printf(SWITCH_CHANNEL_SESSION_LOG(session), SWITCH_LOG_CRIT, "Setting NAT mode based on %s\n", is_nat);
+				}
 
 
 				if (app && data && !strcasecmp(app, "conference")) {