Plane: Protect against a divide by 0 when calculating the forward throttle compensation

ArduPlane/servos.cpp

@@ -361,6 +361,11 @@ void Plane::throttle_voltage_comp()
     // constrain read voltage to min and max params
     batt_voltage_resting_estimate = constrain_float(batt_voltage_resting_estimate,g2.fwd_thr_batt_voltage_min,g2.fwd_thr_batt_voltage_max);
 
+    // don't apply compensation if the voltage is excessively low
+    if (batt_voltage_resting_estimate < 1) {
+        return;
+    }
+
     // Scale the throttle up to compensate for voltage drop
     // Ratio = 1 when voltage = voltage max, ratio increases as voltage drops
     const float ratio = g2.fwd_thr_batt_voltage_max / batt_voltage_resting_estimate;