Fix Buffering length edge cases

[OskarEichler]

Summary

Align OpenSSL::Buffering read length handling with IO: reject negative read sizes, return an empty String for gets(..., 0), and stop filling once a positive gets limit is available.

Reproduction

The current implementation accepts negative lengths in read, readpartial, and read_nonblock; gets(separator, 0) reads through separator/EOF and returns nil at EOF; and a positive limited gets continues reading until separator/EOF after the requested bytes are buffered.

Verification

• Current candidate: 630 tests / 4,624 assertions, zero failures/errors, two expected FIPS omissions
• Release candidate: 597 tests / 4,411 assertions, zero failures/errors, two expected FIPS omissions
• Focused IO-parity and bounded-read models pass
• Native build succeeds with -Werror; gem build/install and Songstats Rails/OpenSSL integration pass

No public API is added; this corrects documented IO-compatible edge behavior.