Fix syntax error

Author: Phlosioneer

src/libcore/char.rs

@@ -905,7 +905,7 @@ impl<I: Iterator<Item = u8>> Iterator for DecodeUtf8<I> {
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let len = self.iter.len();
+        let len = self.0.len();
 
         // A code point is at most 4 bytes long.
         let min_code_points = len / 4;