lightningd: fix crash when we try to send fail_htlc msg to onchaind.

Great report from whitslack on this crash at startup:

```
2020-10-07T13:03:21.419Z **BROKEN** lightningd: FATAL SIGNAL 6 (version 0.9.1)
2020-10-07T13:03:21.419Z **BROKEN** lightningd: backtrace: common/daemon.c:51 (crashdump) 0x559fb67bcc76
2020-10-07T13:03:21.419Z **BROKEN** lightningd: backtrace: /var/tmp/portage/sys-libs/glibc-2.32-r2/work/glibc-2.32/signal/../sysdeps/unix/sysv/linux/x86_64/sigaction.c:0 ((null)) 0x7f61cdca8baf
2020-10-07T13:03:21.419Z **BROKEN** lightningd: backtrace: ../sysdeps/unix/sysv/linux/raise.c:50 (__GI_raise) 0x7f61cdca8b31
2020-10-07T13:03:21.419Z **BROKEN** lightningd: backtrace: /var/tmp/portage/sys-libs/glibc-2.32-r2/work/glibc-2.32/stdlib/abort.c:79 (__GI_abort) 0x7f61cdc92535
2020-10-07T13:03:21.419Z **BROKEN** lightningd: backtrace: /var/tmp/portage/sys-libs/glibc-2.32-r2/work/glibc-2.32/assert/assert.c:92 (__assert_fail_base) 0x7f61cdc9241e
2020-10-07T13:03:21.419Z **BROKEN** lightningd: backtrace: /var/tmp/portage/sys-libs/glibc-2.32-r2/work/glibc-2.32/assert/assert.c:101 (__GI___assert_fail) 0x7f61cdca1241
2020-10-07T13:03:21.419Z **BROKEN** lightningd: backtrace: lightningd/subd.c:750 (subd_send_msg) 0x559fb67a1c31
2020-10-07T13:03:21.419Z **BROKEN** lightningd: backtrace: lightningd/subd.c:745 (subd_send_msg) 0x559fb67a1c31
2020-10-07T13:03:21.419Z **BROKEN** lightningd: backtrace: lightningd/peer_htlcs.c:252 (local_fail_in_htlc) 0x559fb6798f77
2020-10-07T13:03:21.419Z **BROKEN** lightningd: backtrace: lightningd/peer_htlcs.c:1441 (onchain_failed_our_htlc) 0x559fb6798f77
2020-10-07T13:03:21.419Z **BROKEN** lightningd: backtrace: lightningd/onchain_control.c:339 (handle_missing_htlc_output) 0x559fb6786b9d
2020-10-07T13:03:21.419Z **BROKEN** lightningd: backtrace: lightningd/onchain_control.c:455 (onchain_msg) 0x559fb6786b9d
```

The problem is a channel with an onchaind can be in state FUNDING_STATE_SEEN,
because onchaind has started but not responded to init yet (which it does once it
has analyzed the commitment tx).

Channel B is onchain, and its onchaind fails the HTLC, and we try to send a msg
to channel A's onchaind as if it were channeld.

Explicitly check if it's channeld, rather than trying to see if it's onchaind.

Fixes: #4114
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: crash: assertion fail at restart when source and destination channels of an HTLC are both onchain.

lightningd/peer_htlcs.c

@@ -141,7 +141,7 @@ static void tell_channeld_htlc_failed(const struct htlc_in *hin,
 		return;
 
 	/* onchaind doesn't care, it can't do anything but wait */
-	if (channel_on_chain(hin->key.channel))
+	if (!channel_active(hin->key.channel))
 		return;
 
 	subd_send_msg(hin->key.channel->owner,