encoding/asn1: Reject invalid INTEGERs.

encoding/asn1: Reject invalid INTEGERs.

The empty string is not a valid DER integer. DER also requires that values be
minimally-encoded, so excess padding with leading 0s (0xff for negative
numbers) is forbidden. (These rules also apply to BER, incidentally.)

Fixes #12622.

Change-Id: I041f94e34a8afa29dbf94dd8fc450944bc91c9c3
Reviewed-on: https://go-review.googlesource.com/17008
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>