Problem: The server crashes when explaining a query with an IN
subquery with a GROUP BY clause with a subquery selecting a column
that doesn't exist.

During JOIN::prepare() of the IN subquery,
remove_redundant_subquery_clauses() is called to remove redundant
clauses. The GROUP BY clause is found to be redundant, and is
removed. Since it is removed, the subquery with the non-existent
column is not resolved in this round.

Later, all subqueries are explained, also the subquery that was
removed and not prepared. JOIN::prepare is called for this subquery,
but resolving of the non-existent column name crashes in
Item_field::fix_outer_field() when trying to find the Item_subselect
for the middle query. This item has been removed in a transformation
from IN to semijoin.

Fix: Resolve all items before calling
remove_redundant_subquery_clauses(). This has the added benefit of
emitting error messages for non-existent columns.