fix: closer set to remote for onchain settlement

Incase we have been offline while a channel was force closed on us we
now set the 'closer' to 'remote' instead of null because this is by far
the most probable reason.

Changelog-None

lightningd/channel.c

@@ -437,6 +437,7 @@ void channel_set_state(struct channel *channel,
 		if (reason == REASON_LOCAL)   channel->closer = LOCAL;
 		if (reason == REASON_USER)    channel->closer = LOCAL;
 		if (reason == REASON_REMOTE)  channel->closer = REMOTE;
+		if (reason == REASON_ONCHAIN) channel->closer = REMOTE;
 	}
 
 	/* use or update state_change_cause, if known */