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

src/backend/access/spgist/spgscan.c

@@ -56,7 +56,7 @@ pairingheap_SpGistSearchItem_cmp(const pairingheap_node *a,
 	else
 	{
 		/* Order according to distance comparison */
-		for (i = 0; i < so->numberOfOrderBys; i++)
+		for (i = 0; i < so->numberOfNonNullOrderBys; i++)
 		{
 			if (isnan(sa->distances[i]) && isnan(sb->distances[i]))
 				continue;		/* NaN == NaN */