pset: fix slicing in KeySource::deserialize

Lol, this code was so close and yet so far.

Author: apoelstra

src/pset/serialize.rs

@@ -17,7 +17,6 @@
 //! Defines traits used for (de)serializing PSET values into/from raw
 //! bytes in PSET key-value pairs.
 
-use std::convert::TryFrom;
 use std::io;
 
 use crate::confidential::{self, AssetBlindingFactor};
@@ -29,6 +28,7 @@ use crate::{AssetId, BlockHash, Script, Transaction, TxOut, Txid};
 use bitcoin;
 use bitcoin::bip32::{ChildNumber, Fingerprint, KeySource};
 use bitcoin::{key::XOnlyPublicKey, PublicKey};
+use internals::slice::SliceExt;
 use secp256k1_zkp::{self, RangeProof, SurjectionProof, Tweak};
 
 use super::map::{PsbtSighashType, TapTree};
@@ -176,16 +176,15 @@ impl Serialize for KeySource {
 
 impl Deserialize for KeySource {
     fn deserialize(bytes: &[u8]) -> Result<Self, encode::Error> {
-        let Ok(prefix) = <[u8; 4]>::try_from(&bytes[0..4]) else {
+        let Some((prefix, mut rest)) = SliceExt::split_first_chunk::<4>(bytes) else {
             return Err(io::Error::from(io::ErrorKind::UnexpectedEof).into());
         };
 
         let fprint: Fingerprint = Fingerprint::from(prefix);
         let mut dpath: Vec<ChildNumber> = Vec::default();
 
-        let mut d = &bytes[4..];
-        while !d.is_empty() {
-            let index = u32::consensus_decode(&mut d)?;
+        while !rest.is_empty() {
+            let index = u32::consensus_decode(&mut rest)?;
             dpath.push(index.into());
         }