Reject zero digest_size in blake2b generichash bindings

[00200200]

Summary

generichash_blake2b_final() aborts the interpreter with SIGABRT when called on a Blake2State whose digest_size is 0. The same invalid size also causes a poor failure mode through generichash_blake2b_init() / generichash_blake2b_salt_personal() (RuntimeError or abort) because _checkparams() only enforced an upper bound.

libsodium's blake2b_final() calls sodium_misuse() when outlen is 0 (!outlen || outlen > BLAKE2B_OUTBYTES). This is the zero-length counterpart of #964 (oversized digest); #965 covers the upper bound.

Fixes #967.

Fix

• Reject digest_size < 1 in _checkparams() so init/one-shot paths raise ValueError.
• Reject state.digest_size < 1 in generichash_blake2b_final() so a directly constructed Blake2State(0) raises ValueError instead of aborting.

Sizes 1..BYTES_MAX (including values below BYTES_MIN) remain accepted, matching current libsodium C behavior.

Testing

• Added regression tests in tests/test_generichash.py:
  • test_generichash_blake2b_final_rejects_zero_directly_constructed_state
  • test_generichash_blake2b_init_rejects_zero_digest_size
  • test_generichash_blake2b_salt_personal_rejects_zero_digest_size
• On main, generichash_blake2b_final(Blake2State(0)) exits with SIGABRT; with this change it raises ValueError: Digest_size less than 1.
• Ran: PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 pytest tests/test_generichash.py::test_generichash_blake2b_final_rejects_zero_directly_constructed_state tests/test_generichash.py::test_generichash_blake2b_init_rejects_zero_digest_size tests/test_generichash.py::test_generichash_blake2b_salt_personal_rejects_zero_digest_size → passed.