FlightTaskOrbit: fix acceleration limit inverting velocity

The formula only calculated the absolute value of the velocity and
the sign (rotation direction) got lost in that case.

src/lib/FlightTasks/tasks/FlightTaskOrbit.cpp

@@ -60,9 +60,9 @@ bool FlightTaskOrbit::applyCommandParameters(const vehicle_command_s &command)
 bool FlightTaskOrbit::setRadius(const float r)
 {
 	if (math::isInRange(r, _radius_min, _radius_max)) {
-		// radius is more important than velocity for safety
+		// small radius is more important than high velocity for safety
 		if (!checkAcceleration(r, _v, _acceleration_max)) {
-			_v = sqrtf(_acceleration_max * r);
+			_v = math::sign(_v) * sqrtf(_acceleration_max * r);
 		}
 
 		_r = r;