Plane: don't allow a zero speed_scaler

this fixes an issue with yaw damper on quadplanes when at zero
airspeed

Thanks to Vladi Portnoy for reporting this

ArduPlane/Attitude.cpp

@@ -26,7 +26,7 @@ float Plane::get_speed_scaler(void)
             // when in VTOL modes limit surface movement at low speed to prevent instability
             float threshold = aparm.airspeed_min * 0.5;
             if (aspeed < threshold) {
-                float new_scaler = linear_interpolate(0, g.scaling_speed / threshold, aspeed, 0, threshold);
+                float new_scaler = linear_interpolate(0.001, g.scaling_speed / threshold, aspeed, 0, threshold);
                 speed_scaler = MIN(speed_scaler, new_scaler);
 
                 // we also decay the integrator to prevent an integrator from before