internal/socket, ipv4, ipv6: avoid panic on empty control message on non-Unix

On platforms without cmsghdr support (such as Windows), controlHeaderLen
returns 0. Guard against controlHeaderLen() <= 0 in ControlMessage
methods so parsing or marshaling an empty buffer does not index &m[0]
and panic.

Change-Id: I16f8bc821b92a015af4ef9711e7378206a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/852826
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Chase <drchase@google.com>