Allow out_present to be NULL in CBS_get_optional_asn1

This is useful to skip an optional element, and mirrors the behaviour of
CBS_get_optional_asn1_octet_string.

Change-Id: Icb538c5e99a1d4e46412cae3c438184a94fab339
Reviewed-on: https://boringssl-review.googlesource.com/5800
Reviewed-by: Adam Langley <agl@google.com>
2 files changed