The empty contents are not a valid ASN.1 INTEGER.

Zero is encoded as a single zero octet. Per X.690, 8.3.1:

  The encoding of an integer value shall be primitive. The contents octets
  shall consist of one or more octets.

Change-Id: If4304a2be5117b71446a3a62a2b8a6124f85a202
Reviewed-on: https://boringssl-review.googlesource.com/2010
Reviewed-by: Adam Langley <agl@google.com>