ui.cc: fix divide by zero

selfdrive/ui/ui.cc

@@ -162,11 +162,16 @@ static void update_sockets(UIState *s) {
   if (sm.updated("ubloxGnss")) {
     auto data = sm["ubloxGnss"].getUbloxGnss();
     if (data.which() == cereal::UbloxGnss::MEASUREMENT_REPORT) {
+
       auto measurements = data.getMeasurementReport().getMeasurements();
+      if (measurements.size()){
         for (auto m : measurements) {
           scene.cnoAvg += m.getCno();
         }
         scene.cnoAvg /= measurements.size();
+      } else {
+        scene.cnoAvg = 0;
+      }
       scene.satelliteCount = data.getMeasurementReport().getNumMeas();
     }
   }