MDEV-16904 inline void swap(base_list &rhs) should swap list only when list is...

not empty

We should swap the list only when list is not empty.

sql/sql_list.h

@@ -306,10 +306,13 @@ class base_list :public Sql_alloc
   */
   inline void swap(base_list &rhs)
   {
+    list_node **rhs_last=rhs.last;
     swap_variables(list_node *, first, rhs.first);
-    swap_variables(list_node **, last, rhs.last);
     swap_variables(uint, elements, rhs.elements);
+    rhs.last= last == &first ? &rhs.first : last;
+    last = rhs_last == &rhs.first ? &first : rhs_last;
   }
+
   inline list_node* last_node() { return *last; }
   inline list_node* first_node() { return first;}
   inline void *head() { return first->info; }