Windows stdio handles can be null

See also: https://github.com/rust-lang/rust/issues/88576

In short, Windows stdio functions can return null to mean that there are no handles. The [code here](https://github.com/BurntSushi/winapi-util/blob/8f291c9da272b24699f31e22c521404255462642/src/win.rs#L132) assumes stdio handles are always valid `File` handles. While this is technically an incorrect assumption, it worked out ok until Rust's new "Safe I/O" code was implemented.

A workaround will be implemented upstream but I thought there should be an issue here too, since unfortunately it reached stable, causing `env_logger` to break.

Start with src/win.rs around line 132 and read the linked Rust issue about Windows stdio handles returning null. Trace how those handles are treated as File handles and compare the upstream workaround. Done means env_logger no longer breaks when Windows stdio functions return null.

Written by the indexing model from the issue text.