Fix bug in PSBT `Deserialize` for `XOnlyPublicKey`

During upgrade of `secp256k1` a number of function calls needed to be
rewritten to not use `from_slice` but `from_byte_array`. Unfortunately,
the conversion wasn't correct and caused panics on invalid inputs
rather than errors.

This fixes it simply by calling `try_into` on the entire slice and
converting the error.

bitcoin/src/psbt/serialize.rs

@@ -261,7 +261,7 @@ impl Serialize for XOnlyPublicKey {
 impl Deserialize for XOnlyPublicKey {
     fn deserialize(bytes: &[u8]) -> Result<Self, Error> {
         XOnlyPublicKey::from_byte_array(
-            bytes[..32].try_into().expect("statistically impossible to hit"),
+            bytes.try_into().map_err(|_| Error::InvalidXOnlyPublicKey)?,
         )
         .map_err(|_| Error::InvalidXOnlyPublicKey)
     }