Fix AES-GCM tag length query and the test meant to cover it

Querying OSSL_CIPHER_PARAM_AEAD_TAG with a NULL buffer returned a length
of 0 instead of the tag length. Report ctx->taglen in that case, matching
the IV queries.

The test had a missing "goto err", meaning the test passed only when the
assertion failed. Add the goto.

Both were introduced in #28232

Assisted-by: Claude:claude-fable-5
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Bob Beck <beck@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Merge-date: Mon Sep  7 18:55:12 2026
Merged-from: https://github.com/openssl/openssl/pull/32649