Fix seg fault in ASN1_generate_v3/ASN1_generate_nconf

(Imported from upstream's ac5a110621ca48f0bebd5b4d76d081de403da29e)

Change-Id: I1a0518447716a93038bd95d40abef1d9fb4e5f72
Reviewed-on: https://boringssl-review.googlesource.com/4004
Reviewed-by: Adam Langley <agl@google.com>