Fix cksum oom panic

[killuazoldyck680]

Hey, tried to fix the OOM panic.

Changes Made:

• Added a validation guardrail inside the maybe_sanitize_length function.
• If the parsed bit length exceeds u32::MAX, it now cleanly returns a ChecksumError::InvalidLength instead of allowing the application to panic and crash the terminal.

Tested it locally, and it works perfectly on my machine. Ready for review!

Fixes #12869

[oech3]

If the parsed bit length exceeds u32::MAX, it now cleanly returns a ChecksumError::InvalidLength

It is what GNU never do. Please don't make artifactal restriction. People having many RAM cannot calc it.
We should catch OOM instead.

[killuazoldyck680]

ok, will try to fix that.

[oech3]

For example, the following command should not panic:

$ ulimit -v 5400
$ cksum -a shake128 -l 65536 /dev/null
memory allocation of 32768 bytes failed
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Aborted                    (core dumped) cksum -a shake128 -l 65536 /dev/null

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.