crypto/subtle: don't cast to *uintptr when word size is 0
Casting to a *uintptr is not ok if there isn't at least 8 bytes of
data backing that pointer (on 64-bit archs).
So although we end up making a slice of 0 length with that pointer,
the cast itself doesn't know that.
Instead, bail early if the result is going to be 0 length.
Fixes #59334
Change-Id: Id3c0e09d341d838835c0382cccfb0f71dc3dc7e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/480575
Run-TryBot: Keith Randall <khr@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Bryan Mills <bcmills@google.com>