Core/Movement: fixed logic in selecting current movement generator slot

src/server/game/Movement/MotionMaster.cpp

@@ -152,12 +152,12 @@ void MotionMaster::MovementExpired(bool reset /*= true*/)
 
 MovementSlot MotionMaster::GetCurrentSlot() const
 {
-    if (empty())
-        return MOTION_SLOT_ACTIVE;
-
-    if (topOrNull())
+    if (empty() || !top())
         return MOTION_SLOT_IDLE;
 
+    if (top())
+        return MOTION_SLOT_ACTIVE;
+
     return MAX_MOTION_SLOT;
 }