net: do more faithful conversion from AddrPort to UDPAddr

A UDPAddr with a nil IP is a valid state, representing an AF-agnostic
unspecified address, so checking for addr.IsValid() isn't correct;
remove that, as it's only needed in the UDP rx path where it can be
added. Secondly, forcing everything to be IPv6 also is not correct, and
was likely done when the missing .AsSlice() made doing the right thing
less ergonomic. Fix this by using .AsSlice(), which properly preserves
IP version.

Change-Id: Idd1eaecd4076f32a843f859a0a9802ef98f956d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/361478


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>