Avoid type errors in EAI-related name check logic.

Avoid type errors in EAI-related name check logic.

The incorrectly typed data is read only, used in a compare operation, so
neither remote code execution, nor memory content disclosure were possible.
However, applications performing certificate name checks were vulnerable to
denial of service.

The GENERAL_TYPE data type is a union, and we must take care to access the
correct member, based on `gen->type`, not all the member fields have the same
structure, and a segfault is possible if the wrong member field is read.

The code in question was lightly refactored with the intent to make it more
obviously correct.

Fixes CVE-2024-6119

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>