Refactor ASN1_STRING access to use accessor functions (#1462)

Replace direct access to .data and .length attributes with
ASN1_STRING_get0_data() and ASN1_STRING_length() accessor
functions in the X509Extension._subjectAltNameString() method.

This improves compatibility with OpenSSL's opaque structure
design and follows the pattern used elsewhere in the codebase.

Co-authored-by: Claude <[email protected]>
1 file changed