clippy: use u32::MAX rather than u32::max_value

Author: apoelstra

src/transaction.rs

@@ -99,7 +99,7 @@ impl OutPoint {
     pub fn null() -> OutPoint {
         OutPoint {
             txid: Txid::all_zeros(),
-            vout: u32::max_value(),
+            vout: u32::MAX,
         }
     }