u64->usize overflow in h1 decode on 32bit system

[cpud36]

The code in question:

let to_read = *remaining as usize;
let buf = ready!(body.read_mem(cx, to_read))?;
let num = buf.as_ref().len() as u64;

Here, if it ever happens that *remaining is divisible by usize::MAX, to_read will become 0 and the code will return an error IncompleteBody, which is clearly wrong.

Moreover, even in cases when remaining is not exactly divisible by usize::MAX, but has small remainder, this is suboptimal (and likely to produce remainder divisible by usize::MAX).

Maybe it is worth to replace this with saturating cast?

[seanmonstar]

Thanks for the report, good catch. I've put up a fix in #4056.