splice: Clean up warning message

[ddustin]

Add a clean warning message if peer sends junk ACK data.

[Andezion]

The idea is right and matches the existing error-handling pattern in this function. Should splice_accepter() get a symmetric guard?

[Andezion]

Didn't the check placed too late to do anything? Look at the top of the function -

static void splice_initiator(struct peer *peer, const u8 *inmsg)
{
	/ some code /
	struct wally_psbt *psbt = peer->splicing->current_psbt;   - on line 4412
	/ some code /
	type = fromwire_peektype(inmsg);  - on line 4418

	if (!peer->splicing) {  - and here we have new  check
		peer_failed_warn(...);
	}
}

So psbt is initialized from peer->splicing->current_psbt in the declaration block, which executes before type = fromwire_peektype(inmsg) and before the new if (!peer->splicing) check. If peer->splicing is NULL, the function already dereferences it at line 4412 and crashes - so the new check is unreachable code?

Maybe we should move the NULL check to the very first line of the function, before psbt is declared/initialized?