ssl: Use accessors for ASN1_STRING fields

[davidben]

Filing this for a patch I'll upload shortly. See openssl/openssl#29117.

Linked PRs

• gh-141801: Use accessors for ASN1_STRING fields #141802
• [3.14] gh-141801: Use accessors for ASN1_STRING fields in libssl (GH-141802) #141847
• [3.13] gh-141801: Use accessors for ASN1_STRING fields in libssl (GH-141802) #141848

[gpshead]

thanks!