Prevents denial of service attacks from bogus UDP packets.

Fixes #13281.

Change-Id: Ifb51b17a1b0807bfd27b144d6037431701184e7b
Reviewed-on: https://go-review.googlesource.com/22126Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>