qt ui: fix segfault if networkmanager is not running

selfdrive/ui/qt/offroad/wifi.cc

@@ -155,6 +155,10 @@ void Networking::successfulConnection(QString ssid) {
 }
 
 void Networking::sidebarChange(){
+  if (s == nullptr || an == nullptr){
+    return;
+  }
+
   s->setCurrentIndex(1);
   an->s->setCurrentIndex(1);
   refresh();

selfdrive/ui/qt/offroad/wifi.hpp

@@ -72,11 +72,11 @@ public:
   explicit Networking(QWidget* parent = 0);
 
 private:
-  QStackedLayout* s;// keyboard, wifiScreen, advanced
+  QStackedLayout* s = nullptr;// keyboard, wifiScreen, advanced
+  AdvancedNetworking* an = nullptr;
 
   Network selectedNetwork;
 
-  AdvancedNetworking* an;
   WifiUI* wifiWidget;
   WifiManager* wifi = nullptr;
   InputField* inputField;