Fixing DoS regression problem.

Using "key_part->fieldnr - 1" to access the desired field
is only correct in real INSERT queries.
In case of inserting records into a temporary table
when performing GROUP BY queries this expression does not work.

Fix: Instead of accessing field_length and comparing it
to key_part->length, there is an easier way to check if
we're dealing with a prefix key: check key_part_flag against
HA_PART_KEY_SEG flag.