checksum: Fix EOF handling

Using `Read::read_exact` is not the correct way to read a file in
chunks.  When it returns `UnexpectedEof`, the buffer has data in it, but
is zero-padded to the end.  This causes the calculated checksum to be
incorrect.

src/main.rs

@@ -303,14 +303,11 @@ fn checksum(path: impl AsRef<Path>) -> std::io::Result<Vec<u8>> {
     let mut blake = Blake2b512::new();
     loop {
         let mut buf = vec![0u8; 16384];
-        match f.read_exact(&mut buf) {
-            Ok(_) => blake.update(buf),
-            Err(e) if e.kind() == std::io::ErrorKind::UnexpectedEof => {
-                blake.update(buf);
+        let sz = f.read(&mut buf)?;
+        if sz == 0 {
             break;
         }
-            Err(e) => return Err(e),
-        }
+        blake.update(&buf[..sz]);
     }
     Ok(blake.finalize().to_vec())
 }