Incorrect upper bound for `size_hint` of `char::DecodeUtf16`

[steffahn]

use std::char::decode_utf16;

fn main() {
    let x = [0xD834, 0x006d];
    let mut d = decode_utf16(x);
    d.next();
    let (_, u) = d.size_hint();
    let c = d.count();
    println!("upper bound: {:?}, actual length {}", u, c);
    assert!(u.unwrap() >= c);
}

   Compiling playground v0.0.1 (/playground)
    Finished dev [unoptimized + debuginfo] target(s) in 1.04s
     Running `target/debug/playground`
thread 'main' panicked at 'assertion failed: u.unwrap() >= c', src/main.rs:10:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Standard Output

upper bound: Some(0), actual length 1

(playground)

@rustbot label T-libs, A-iterators