gossip_control: when searching for a txout, make sure it's not spent!

There's no reason for the db to ever return non-NULL if it's spent.  And there's
only one caller, for which that is definitely true.

Suggested-by: @cdecker
Fixes: #1934
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_gossip.py

@@ -4,7 +4,6 @@ from utils import wait_for, TIMEOUT, only_one
 import json
 import logging
 import os
-import pytest
 import struct
 import subprocess
 import time
@@ -865,7 +864,6 @@ def test_gossipwith(node_factory):
     assert num_msgs == 5
 
 
-@pytest.mark.xfail(strict=True)
 def test_gossip_notices_close(node_factory, bitcoind):
     # We want IO logging so we can replay a channel_announce to l1.
     l1 = node_factory.get_node(options={'log-level': 'io'})

wallet/wallet.c

@@ -2139,7 +2139,8 @@ struct outpoint *wallet_outpoint_for_scid(struct wallet *w, tal_t *ctx,
 			  "FROM utxoset "
 			  "WHERE blockheight = ?"
 			  " AND txindex = ?"
-			  " AND outnum = ?");
+			  " AND outnum = ?"
+			  " AND spendheight IS NULL");
 	sqlite3_bind_int(stmt, 1, short_channel_id_blocknum(scid));
 	sqlite3_bind_int(stmt, 2, short_channel_id_txnum(scid));
 	sqlite3_bind_int(stmt, 3, short_channel_id_outnum(scid));