object-refs: avoid division by zero

Currently, we don't check refs_hash_size size and happily call
lookup_object_refs() even if refs_hash_size is zero which leads to
a division by zero in hash_obj().

Signed-off-by: Andre Noll <maan@systemlinux.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

object-refs.c

@@ -127,6 +127,9 @@ void mark_reachable(struct object *obj, unsigned int mask)
 
 	if (!track_object_refs)
 		die("cannot do reachability with object refs turned off");
+	/* nothing to lookup */
+	if (!refs_hash_size)
+		return;
 	/* If we've been here already, don't bother */
 	if (obj->flags & mask)
 		return;