Fix division by zero in _bt_vacuum_needs_cleanup()

Checks inside _bt_vacuum_needs_cleanup() allow division by zero to happen when
metad->btm_last_cleanup_num_heap_tuples == 0.  This commit adjusts the
expression so that no division by zero might happen.

Reported-by: Piotr Stefaniak
Discussion: https://postgr.es/m/DB8PR03MB5931C41F7787A95313F08322F22A0%40DB8PR03MB5931.eurprd03.prod.outlook.com
Reviewed-by: Masahiko Sawada
Backpatch-through: 11

src/backend/access/nbtree/nbtree.c

@@ -835,7 +835,7 @@ _bt_vacuum_needs_cleanup(IndexVacuumInfo *info)
 		prev_num_heap_tuples = metad->btm_last_cleanup_num_heap_tuples;
 
 		if (cleanup_scale_factor <= 0 ||
-			prev_num_heap_tuples < 0 ||
+			prev_num_heap_tuples <= 0 ||
 			(info->num_heap_tuples - prev_num_heap_tuples) /
 			prev_num_heap_tuples >= cleanup_scale_factor)
 			result = true;