crypto/x509: permit serial numbers to be negative.

crypto/x509: permit serial numbers to be negative.

Some software that produces certificates doesn't encode integers
correctly and, about half the time, ends up producing certificates with
serial numbers that are actually negative.

This buggy software, sadly, appears to be common enough that we should
let these errors pass. This change allows a Certificate.SerialNumber to
be negative.

Fixes #8265.

Change-Id: Ief35dae23988fb6d5e2873e3c521366fb03c6af4
Reviewed-on: https://go-review.googlesource.com/17247
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>