crypto/x509: truncate signed hash before DSA signature verification

According to spec, the hash must be truncated, but crypto/dsa
does not do it. We can't fix it in crypto/dsa, because it would break
verification of previously generated signatures.
In crypto/x509 however, go can't generate DSA certs, only verify them,
so the fix here should be safe.

Fixes #22017

Change-Id: Iee7e20a5d76f45da8901a7ca686063639092949f
GitHub-Last-Rev: 8041cde8d25d3a336b81d86bd52bff5039568246
GitHub-Pull-Request: golang/go#34630
Reviewed-on: https://go-review.googlesource.com/c/go/+/198138


Reviewed-by: Filippo Valsorda <filippo@golang.org>