FUNCTION IN SUBQUERY

ANALYSIS:
=========
Server runs into null pointer dereferencing when a
deterministic function returning longtext is used in a
subquery.

Item::const_item() is 'true' for all items having no table
reference bits set. So it is 'true' for subqueries and
deterministic functions that do not reference any tables.
However, we cannot always consider items to be constant
based on the referenced table bits. In this scenario, a
deterministic function is considered a const item. Hence the
resolver tries to evaluate the function in the preparation
phase. At this phase, the query tables are open but not
locked. After the evaluation, all the tables in the outer
query block are closed. Later during the execution phase,
when we try to lock the query tables, the TABLE object will
be unused, thus leading to a null pointer dereference.

FIX:
====
We are overriding the function 'fix_after_pullout' in the
'Item_func_sp' class where we are not considering
deterministic functions as const items based on their table
reference, thus preventing the resolver from evaluating
the function in the preparation phase.