RADIUS client: Print a clear debug log entry if socket is not available

It could have been possible to select a socket that is not open
(sel_sock == -1) and try to use that in socket operations. This would
fail with potentially confusing error messages. Make this clearer by
printing a clear debug log entry on socket not being available.
(CID 72696)

Signed-off-by: Jouni Malinen <j@w1.fi>

src/radius/radius_client.c

@@ -1039,6 +1039,13 @@ radius_change_server(struct radius_client_data *radius,
 		return -1;
 	}
 
+	if (sel_sock < 0) {
+		wpa_printf(MSG_INFO,
+			   "RADIUS: No server socket available (af=%d sock=%d sock6=%d auth=%d",
+			   nserv->addr.af, sock, sock6, auth);
+		return -1;
+	}
+
 	if (conf->force_client_addr) {
 		switch (conf->client_addr.af) {
 		case AF_INET: