We have several implementations of this internally, so consolidate them.
Chromium also has a copy in net/der/parse_values.cc which could call
into this.
(I'm also hoping we can make c2i_ASN1_INTEGER call this and
further tighten up crypto/asn1's parser, but I see Chromium still has an
allow_invalid_serial_numbers option, so perhaps not quite yet.)
Update-Note: This CL does not change behavior, but I'm leaving a note to
myself to make net/der/parse_values.cc call the new functions.
Change-Id: If2aae6574ba6a30e343e1308da6af543616156ec
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/44051
Reviewed-by: Adam Langley <agl@google.com>