Fixed a crash in Item_equal::fix_length_and_dec that was a result of the
architecture of the MWL#89 subquery optimization.

The injection of IN->EXISTS predicates for subqueries happened inside
make_join_statistics, after constant substitution, but before multiple
equality substitution done by substitute_for_best_equal_field. As a
result, when we called fix_fields for the WHERE clause after the injection
of IN->EXISTS predicates, Item_equal was not fixed, and it was in a state
not anticipated by Item_equal::fix_length_and_dec - the Item_equal
containted only a constant, and no fields at all.

The fix takes into account this new possible state when calling fix_fields.