fix(http1): fix reading large bodies on 32-bit systems

[seanmonstar]

On 32-bit systems, the cast of the length to read from a u64 to a usize could
truncate, and cause less to be read. If the length was larger than usize::MAX,
the end could not be read.

To fix this, we use a saturating cast pattern instead.

Closes #4055

[cpud36]

In extremely rare cases

Although it is not important, I believe it is guaranteed to happen if ever remaining becomes larger than usize::MAX. The code can never decrease remaining more than to_read, thus it cannot pass below the usize::MAX boundary...

[seanmonstar]

You're right, I stepped through a bit more. I've adjusted the language and title.