commit-reach: avoid NULL dereference

The loop at the top of can_all_from_reach_with_flag() already
accounts for `from->objects[i].item' being NULL, so it follows
the cleanup loop should also account for a NULL `from_one'.

I managed to segfault here on one of my giant, many-remote repos
using `git fetch --negotiation-tip=...  --negotiation-only'
where the --negotiation-tip= argument was a glob which (inadvertently)
captured more refs than I wanted.  I have not reproduced this
in a standalone test case.

Signed-off-by: Eric Wong <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Eric Wong

commit-reach.c

@@ -628,8 +628,12 @@ int can_all_from_reach_with_flag(struct object_array *from,
 	}
 	free(list);
 
-	for (i = 0; i < from->nr; i++)
-		from->objects[i].item->flags &= ~assign_flag;
+	for (i = 0; i < from->nr; i++) {
+		struct object *from_one = from->objects[i].item;
+
+		if (from_one)
+			from_one->flags &= ~assign_flag;
+	}
 
 	return result;
 }