AP_Motors: Check for battery voltage reading fail

libraries/AP_Motors/AP_Motors_Class.cpp

@@ -276,15 +276,15 @@ int16_t AP_Motors::apply_thrust_curve_and_volt_scaling(int16_t pwm_out, int16_t
 void AP_Motors::update_lift_max_from_batt_voltage()
 {
     // sanity check battery_voltage_min is not too small
-    _batt_voltage_min = max(_batt_voltage_min, _batt_voltage_max * 0.6f);
-
     // if disabled or misconfigured exit immediately
-    if((_batt_voltage_max <= 0) || (_batt_voltage_min >= _batt_voltage_max)) {
+    if((_batt_voltage_max <= 0) || (_batt_voltage_min >= _batt_voltage_max) || (_batt_voltage < 0.25*_batt_voltage_min)) {
         _batt_voltage_filt.reset(1.0f);
         _lift_max = 1.0f;
         return;
     }
 
+    _batt_voltage_min = max(_batt_voltage_min, _batt_voltage_max * 0.6f);
+
     // add current based voltage sag to battery voltage
     float batt_voltage = _batt_voltage + _batt_current * _batt_resistance;
     batt_voltage = constrain_float(batt_voltage, _batt_voltage_min, _batt_voltage_max);