AC_PosControl: Enable altitude limit checking.

libraries/AC_AttitudeControl/AC_PosControl.cpp

@@ -137,8 +137,8 @@ void AC_PosControl::set_alt_target_with_slew(float alt_cm, float dt)
 void AC_PosControl::set_alt_target_from_climb_rate(float climb_rate_cms, float dt, bool force_descend)
 {
     // adjust desired alt if motors have not hit their limits
-    // To-Do: add check of _limit.pos_up and _limit.pos_down?
+    // To-Do: add check of _limit.pos_down?
-    if ((climb_rate_cms<0 && (!_motors.limit.throttle_lower || force_descend)) || (climb_rate_cms>0 && !_motors.limit.throttle_upper)) {
+    if ((climb_rate_cms<0 && (!_motors.limit.throttle_lower || force_descend)) || (climb_rate_cms>0 && !_motors.limit.throttle_upper && !_limit.pos_up)) {
         _pos_target.z += climb_rate_cms * dt;
     }