Before this patch the crash occured when a single row dataset is used and
Item::remove_eq_conds() is called for HAVING. This function is not supposed
to be called after the elimination of multiple equalities.

To fix this problem instead of Item::remove_eq_conds() Item::val_int() is
used. In this case the optimizer tries to evaluate the condition for the
single row dataset and discovers impossible HAVING immediately. So, the
execution phase is skipped.

Approved by Igor Babaev <igor@maridb.com>