Fix compiler warnings in MSVC for winreg.c

[Fidget-Spinner]

During compilation, I get a warning:
1>\Documents\GitHub\cpython\PC\winreg.c(683,21): warning C4018: '<': signed/unsigned mismatch.
Caused by the line assert(len < size); because len is type Py_ssize_t (signed) and size is type DWORD (unsigned).

We can cast len to unsigned safely to silence the warning (which is what the compiler already does implicitly), because there is already a check for assert(len >= 0); above.

[Fidget-Spinner]

I think this warrants a skip news and skip issue, asserts don't affect release builds AFAIK.

I separated this from #24440 because maybe this needs a backport while that one doesn't.

[zooba]

Thanks! I'll do an auto backport and see how far it goes, but it's not essential.

[miss-islington]

Thanks @Fidget-Spinner for the PR, and @zooba for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8, 3.9.
🐍🍒⛏🤖

[miss-islington]

Sorry, @Fidget-Spinner and @zooba, I could not cleanly backport this to 3.9 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 28873a70503c9e700fe05c3f9c781dbd6ea19b14 3.9

[miss-islington]

Sorry @Fidget-Spinner and @zooba, I had trouble checking out the 3.8 backport branch.
Please backport using cherry_picker on command line.
cherry_picker 28873a70503c9e700fe05c3f9c781dbd6ea19b14 3.8

[zooba]

Didn't backport cleanly, so we'll just leave it.