UAS: Interpret UINT16_MAX as unknown for battery voltage.

src/uas/UAS.cc

@@ -495,6 +495,7 @@ void UAS::receiveMessage(mavlink_message_t message)
             emit loadChanged(this,state.load/10.0f);
             emit valueChanged(uasId, name.arg("load"), "%", state.load/10.0f, time);
 
+            if (state.voltage_battery != UINT16_MAX) {
                 // Battery charge/time remaining/voltage calculations
                 currentVoltage = state.voltage_battery/1000.0f;
                 lpVoltage = filterVoltage(currentVoltage);
@@ -525,6 +526,8 @@ void UAS::receiveMessage(mavlink_message_t message)
                 chargeLevel = state.battery_remaining;
 
                 emit batteryChanged(this, lpVoltage, currentCurrent, getChargeLevel(), 0);
+            }
+
             emit valueChanged(uasId, name.arg("battery_remaining"), "%", getChargeLevel(), time);
             emit valueChanged(uasId, name.arg("battery_voltage"), "V", currentVoltage, time);