Removed indirect reference in equalities for cache index lookup.

We should use a direct reference because some optimization of the
query may optimize out a condition predicate and if the outer reference
is the only element of the condition predicate the indirect reference
becomes NULL.

We can resolve correctly the indirect reference in
Expression_cache_tmptable::make_equalities because it is called before
optimization of the cached subquery.


mysql-test/r/subquery_cache.result:
  The test suite for the bug added.
mysql-test/t/subquery_cache.test:
  The test suite for the bug added.
sql/sql_expression_cache.cc:
  Removed indirect reference in equalities for cache index lookup.