AP_MotorsHeli: constrain filtered throttle

This is required because we have removed the constraint on the throttle input.  This also insures that there is no lag caused by the filtered throttle straying far outside the 0 to 1 range

libraries/AP_Motors/AP_MotorsHeli.cpp

@@ -356,6 +356,14 @@ void AP_MotorsHeli::reset_swash_servo(RC_Channel& servo)
 void AP_MotorsHeli::update_throttle_filter()
 {
     _throttle_filter.apply(_throttle_in, 1.0f/_loop_rate);
+
+    // constrain filtered throttle
+    if (_throttle_filter.get() < 0.0f) {
+        _throttle_filter.reset(0.0f);
+    }
+    if (_throttle_filter.get() > 1.0f) {
+        _throttle_filter.reset(1.0f);
+    }
 }
 
 // reset_flight_controls - resets all controls and scalars to flight status