Fix usage of aggregate pathkeys in group_keys_reorder_by_pathkeys()

Fix usage of aggregate pathkeys in group_keys_reorder_by_pathkeys()

group_keys_reorder_by_pathkeys() function searched for matching pathkeys
within root->group_pathkeys.  That could lead to picking an aggregate pathkey
and using its pathkey->pk_eclass->ec_sortref as an argument of
get_sortgroupref_clause_noerr().  Given that ec_sortref of an aggregate pathkey
references aggregate targetlist not query targetlist, this leads to incorrect
query optimization.

Fix this by looking for matching pathkeys only within the first
num_groupby_pathkeys pathkeys.

Reported-by: David G. Johnston
Discussion: https://postgr.es/m/CAKFQuwY3Ek%3DcLThgd8FdaSc5JRDVt0FaV00gMcWra%2BTAR4gGUw%40mail.gmail.com
Author: Andrei Lepikhov, Alexander Korotkov