fix overflow

Author: steveklabnik

src/base64.rs

@@ -333,7 +333,8 @@ impl<'a, T: ?Sized + FromBase64> FromBase64 for &'a T {
 ///
 /// ```
 /// let mut ch = 0u8;
-/// loop {
+/// for ch in 0..255 {
+///     let ch = ch as u8;
 ///     let code = match ch {
 ///         b'A'...b'Z' => ch - 0x41,
 ///         b'a'...b'z' => ch - 0x47,