Don't assume that PageIsEmpty() returns true on an all-zeros page.

It does currently, and I don't see us changing that any time soon, but we
don't make that assumption anywhere else.

Per Tom Lane's suggestion. Backpatch to 9.2, like the previous patch that
added this assumption.

src/backend/access/spgist/spgvacuum.c

@@ -655,7 +655,7 @@ spgvacuumpage(spgBulkDeleteState *bds, BlockNumber blkno)
 	 */
 	if (!SpGistBlockIsRoot(blkno))
 	{
-		if (PageIsEmpty(page))
+		if (PageIsNew(page) || PageIsEmpty(page))
 		{
 			RecordFreeIndexPage(index, blkno);
 			bds->stats->pages_deleted++;