We mustn't use data from p.header to pre-allocate slices for Message.Question, etc.
Otherwise an attacker can force the allocation of several MiB per parsed message,
which can lead to a DoS via putting pressure on the GC.

Fixes golang/go#23214

Change-Id: I6c99577f625b08331b438533adb6b8167bcd1ec5
Reviewed-on: https://go-review.googlesource.com/85135Reviewed-by: Ian Gudger <igudger@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>