bcachefs: Fix rand_delete() test

When we didn't find a key to delete we were getting a null ptr deref.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/tests.c

@@ -563,15 +563,14 @@ static int __do_delete(struct btree_trans *trans, struct bpos pos)
 
 	iter = bch2_trans_get_iter(trans, BTREE_ID_XATTRS, pos,
 				   BTREE_ITER_INTENT);
-	ret = PTR_ERR_OR_ZERO(iter);
-	if (ret)
-		goto err;
-
 	k = bch2_btree_iter_peek(iter);
 	ret = bkey_err(k);
 	if (ret)
 		goto err;
 
+	if (!k.k)
+		goto err;
+
 	bkey_init(&delete.k);
 	delete.k.p = k.k->p;