Core/Movement: Add safety check to avoid C++ undefined behaviour

(cherry picked from commit 67f28adaf738ebf1377d06adac2fa30033928dd6)

src/server/game/Movement/MotionMaster.cpp

@@ -1112,7 +1112,8 @@ void MotionMaster::Remove(MotionMasterContainer::iterator iterator, bool active,
 
 void MotionMaster::Pop(bool active, bool movementInform)
 {
-    Remove(_generators.begin(), active, movementInform);
+    if (!_generators.empty())
+        Remove(_generators.begin(), active, movementInform);
 }
 
 void MotionMaster::DirectInitialize()