fix: reject hashes containing NUL bytes in compare

[abhu85]

Summary

Reject bcrypt hashes containing NUL bytes (\x00) in compare() and compareSync() to prevent non-canonical hash validation.

Problem

When a hash string contains a NUL byte, the native C++ binding truncates at the first \x00 (via c_str()), causing non-canonical hashes like validHash + "\x00extra" to validate against the password. This is a security concern as it allows hash manipulation.

Solution

Add input validation in compare() and compareSync() to reject hash strings containing NUL bytes before passing to the native binding.

Test Plan

• compare(pw, hash + "\x00extra") rejects
• compareSync(pw, hash + "\x00extra") rejects
• Normal compare/compareSync still works
• Existing test suite passes (81/81)

Fixes #1224