Bug#48157: crash in Item_field::used_tables

MySQL handles the join syntax "JOIN ... USING( field1,
... )" and natural joins by building the same parse tree as
a corresponding join with an "ON t1.field1 = t2.field1 ..."
expression would produce. This parse tree was not cleaned up
properly in the following scenario. If a thread tries to
lock some tables and finds that the tables were dropped and
re-created while waiting for the lock, it cleans up column
references in the statement by means a per-statement free
list. But if the statement was part of a stored procedure,
column references on the stored procedure's free list weren't
cleaned up and thus contained pointers to freed objects.

Fixed by adding a call to clean up the current prepared
statement's free list.

sql/sql_parse.cc

@@ -615,8 +615,10 @@ void free_items(Item *item)
   DBUG_VOID_RETURN;
 }
 
-/* This works because items are allocated with sql_alloc() */
-
+/**
+   This works because items are allocated with sql_alloc().
+   @note The function also handles null pointers (empty list).
+*/
 void cleanup_items(Item *item)
 {
   DBUG_ENTER("cleanup_items");