Fix memory leak when using variables

src/Binder/BoundExpressions/BoundExpression.hpp

@@ -141,7 +141,9 @@ public:
         _key = key;
     }
 
-    ~BoundVariableExpression() override = default;
+    ~BoundVariableExpression() override{
+        delete _key;
+    }
 
     BoundExpressionKind GetKind() final{
         return BoundExpressionKind ::Variable;