Out of bound issues don't return a nice error

[DanGould]

rust-payjoin/payjoin/src/v2.rs

Lines 90 to 91 in d23ed51

	let re = PublicKey::from_slice(&message_b[..33])?;
	let nonce = Nonce::from_slice(&message_b[33..45]);

resulting in a panic:

Sent fallback transaction
thread 'main' panicked at payjoin/src/v2.rs:90:46:
range end index 33 out of range for slice of length 0
range end index 33 out of range for slice of length 0

These should use fallable data indexing and map to a nice error