Backport into build-200909301147-5.0.84sp1

> ------------------------------------------------------------
> revno: 2791.2.3
> revision-id: joro@sun.com-20090827114042-h55n7qp9990bl6ge
> parent: anurag.shekhar@sun.com-20090831073231-e55y1hsck6n08ux8
> committer: Georgi Kodinov <joro@sun.com>
> branch nick: B46749-5.0-bugteam
> timestamp: Thu 2009-08-27 14:40:42 +0300
> message:
>   Bug #46749: Segfault in add_key_fields() with outer subquery level 
>     field references
>   
>   This error requires a combination of factors : 
>   1. An "impossible where" in the outermost SELECT
>   2. An aggregate in the outermost SELECT
>   3. A correlated subquery with a WHERE clause that includes an outer 
>   field reference as a top level WHERE sargable predicate
>   
>   When JOIN::optimize detects an "impossible WHERE" it will bail out
>   without doing the rest of the work and initializations. It will not
>   call make_join_statistics() as well.  And make_join_statistics fills 
>   in various structures for each table referenced.
>   When processing the result of the "impossible WHERE" the query must
>   send a single row of data if there are aggregate functions in it.
>   In this case the server marks all the aggregates as having received 
>   no rows and calls the relevant Item::val_xxx() method on the SELECT
>   list. However if this SELECT list happens to contain a correlated 
>   subquery this subquery is evaluated in a normal evaluation mode.
>   And if this correlated subquery has a reference to a field from the 
>   outermost "impossible where" SELECT the add_key_fields will mistakenly
>   consider the outer field reference as a "local" field reference when 
>   looking for sargable predicates.
>   But since the SELECT where the outer field reference refers to is not
>   completely initialized due to the "impossible WHERE" in this level
>   we'll get a NULL pointer reference.
>   Fixed by making a better condition for discovering if a field is "local"
>   to the SELECT level being processed. 
>   It's not enough to look for OUTER_REF_TABLE_BIT in this case since 
>   for outer references to constant tables the Item_field::used_tables() 
>   will return 0 regardless of whether the field reference is from the 
>   local SELECT or not.

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