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.