Call wpas_notify_network_selected only if a specific network was selected

ssid could be NULL here at least based on the function documentation,
so better check whether that is the case prior to calling the
notification function.

wpa_supplicant/wpa_supplicant.c

@@ -1481,7 +1481,8 @@ void wpa_supplicant_select_network(struct wpa_supplicant *wpa_s,
 	wpa_s->reassociate = 1;
 	wpa_supplicant_req_scan(wpa_s, 0, 0);
 
-	wpas_notify_network_selected(wpa_s, ssid);
+	if (ssid)
+		wpas_notify_network_selected(wpa_s, ssid);
 }