crypto/rsa: minor FIPS 186-5 compliance fixes

crypto/rsa: minor FIPS 186-5 compliance fixes

None of these checks actually matter, and indeed we didn't have them
before, but they are required by FIPS 186-5.

Fixes #69799
For #69536

Change-Id: I5e866962a1b2a31a753053e5b9ec50a3f4c87394
Reviewed-on: https://go-review.googlesource.com/c/go/+/632535
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>