nbtree: assert no scheduled primscan between pages.

Follow-up to bugfix commit 763d65ae.  Technically this new assertion is
redundant with the assertion recently added to _bt_readpage by that same
commit, but it seems like a good idea to have both.

The new assertion makes it clear that we expect to call _bt_readnextpage
when there's another primitive index scan scheduled, though only when
needed as the final step of ending the current primitive scan.

src/backend/access/nbtree/nbtsearch.c

@@ -2272,6 +2272,8 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno,
 			return false;
 		}
 
+		Assert(!so->needPrimScan);
+
 		if (ScanDirectionIsForward(dir))
 		{
 			/* read blkno, but check for interrupts first */