wallet: fix case of failed payment not yet in db.

From test_reconnect_sender_add1:

lightningd(13643):BROKEN: backtrace: wallet/wallet.c:1537 (wallet_payment_set_status) 0x561c91b03080
lightningd(13643):BROKEN: backtrace: lightningd/pay.c:67 (payment_failed) 0x561c91ac4f99
lightningd(13643):BROKEN: backtrace: lightningd/peer_htlcs.c:132 (fail_out_htlc) 0x561c91acf627
lightningd(13643):BROKEN: backtrace: lightningd/peer_htlcs.c:321 (hout_subd_died) 0x561c91acfb62

When payment fails, we call wallet_payment_set_status; this is perfectly
possible before it's been committed.

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

wallet/wallet.c

@@ -1397,9 +1397,15 @@ void wallet_payment_set_status(struct wallet *wallet,
 			       const struct preimage *preimage)
 {
 	sqlite3_stmt *stmt;
+	struct wallet_payment *payment;
 
-	/* We should never try this on an unstored payment! */
-	assert(!find_unstored_payment(wallet, payment_hash));
+	/* We can only fail an unstored payment! */
+	payment = find_unstored_payment(wallet, payment_hash);
+	if (payment) {
+		assert(newstatus == PAYMENT_FAILED);
+		tal_free(payment);
+		return;
+	}
 
 	stmt = db_prepare(wallet->db,
 			  "UPDATE payments SET status=? "