Fix bug in pairingheap_SpGistSearchItem_cmp()
Our item contains only so->numberOfNonNullOrderBys of distances. Reflect that
in the loop upper bound.
Discussion: https://postgr.es/m/
53536807-784c-e029-6e92-
6da802ab8d60%40postgrespro.ru
Author: Nikita Glukhov
Backpatch-through: 12