sql: fix off-by-one error in QP

In scope of 8fac6972 it was fixed misbehavior while passing floating
point values to integer iterator. Unfortunately, that patch introduced
off-by-one error. Number of constraints (equalities and inequalities)
can not be greater than number of key parts (obviously, each constraint
can be tested against at most one index part). Inequality constraint can
involve only field representing last key part. To get it number of
constraints was used as index. However, since array is 0-based, the last
key part should be calculated as parts[eq_numb - 1]. Before this patch
it was parts[eq_numb].

Closes #4558

(cherry picked from commit bb905d1d)