gossipd: fix stale scid query.

We always ended up sending an empty query when we had stale scids!
And it turns out we consider such a query invalid:

    Bad query_short_channel_ids query_flags 010506226e46111a0b59caaf126043eb5bbf28c34f3a5e332a1fc7b2b73cf188910f000100010100

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

gossipd/seeker.c

@@ -358,6 +358,7 @@ static struct short_channel_id *stale_scids_remove(const tal_t *ctx,
 		(*query_flags)[i] = *qf;
 		uintmap_del(&seeker->stale_scids, scid);
 		tal_free(qf);
+		i++;
 	}
 	tal_resize(&scids, i);
 	tal_resize(query_flags, i);