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Adds a check in the DnsQuery to ensure we don't parse extra bytes
from the buffer. Also adds a check in DnsResponse that ensures we
don't leak any uninitialized bytes if we allocated the buffer too
large, after constructing the DnsResponse from a DnsQuery.

Bug: 891521
Test: ./net_unittests
Change-Id: Ia0fd4c677be71efec55e452e5219c34c8112a5e5
Reviewed-on: https://chromium-review.googlesource.com/c/1263280
Commit-Queue: Matthew Denton <mpdenton@chromium.org>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600893}