crypto/pbkdf2: add keyLength limit

crypto/pbkdf2: add keyLength limit

As specified by RFC 8018. Also prevent unexpected overflows on 32 bit
systems.

Change-Id: I50c4a177b7d1ebb15f9b3b96e515d93f19d3f68e
Reviewed-on: https://go-review.googlesource.com/c/go/+/644122
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>