sql: assertion fail on nested select

To optimize the select-subquery tarantool uses
subquery flattening optimization which is only used
with respect to some restrictions. When any of
subselects but the last one has an ORDER BY
clause we should raise an error. So subquery
components containing it are not accepted
to be optimized.

Checking part was only checking if the last
subselect contains the clause which led to an
assertion fail. With the patch applied
flattening is not used in case any of
subselects has an ORDER BY.

Closes #3353