Add test checking `XOnlyPublicKey::deserialize`

This test checks the previous fix - if ordered before the previous
commit it will fail.

bitcoin/src/psbt/serialize.rs

@@ -461,6 +461,12 @@ mod tests {
         assert!(sighash.is_ok())
     }
 
+    #[test]
+    fn deserialize_xonly_public_key_len() {
+        assert!(XOnlyPublicKey::deserialize(&[1; 31]).is_err());
+        assert!(XOnlyPublicKey::deserialize(&[1; 33]).is_err());
+    }
+
     #[test]
     #[should_panic(expected = "InvalidMagic")]
     fn invalid_vector_1() {