src/go.mod: import x/crypto/cryptobyte security fix for 32-bit archs

src/go.mod: import x/crypto/cryptobyte security fix for 32-bit archs

	cryptobyte: fix panic due to malformed ASN.1 inputs on 32-bit archs

	When int is 32 bits wide (on 32-bit architectures like 386 and arm), an
	overflow could occur, causing a panic, due to malformed ASN.1 being
	passed to any of the ASN1 methods of String.

	Tested on linux/386 and darwin/amd64.

	This fixes CVE-2020-7919 and was found thanks to the Project Wycheproof
	test vectors.

	Change-Id: I8c9696a8bfad1b40ec877cd740dba3467d66ab54
	Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/645211
	Reviewed-by: Katie Hockman <katiehockman@google.com>
	Reviewed-by: Adam Langley <agl@google.com>
	Reviewed-on: https://go-review.googlesource.com/c/crypto/+/216677
	Run-TryBot: Katie Hockman <katie@golang.org>
	Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
	Reviewed-by: Filippo Valsorda <filippo@golang.org>
	TryBot-Result: Gobot Gobot <gobot@golang.org>

x/crypto/cryptobyte is used in crypto/x509 for parsing certificates.
Malformed certificates might cause a panic during parsing on 32-bit
architectures (like arm and 386).

Change-Id: I840feb54eba880dbb96780ef7adcade073c4c4e3
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/647741
Reviewed-by: Katie Hockman <katiehockman@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/216680
Reviewed-by: Katie Hockman <katie@golang.org>