The {WireBytesRef} constructor checks that {offset + length} does not
overflow. Hence we need to check for illegal sizes before constructing
the {WireBytesRef}.

The {consume_bytes} function already does that, so remove the
redundant hand-written checking.

R=titzer@chromium.org

Bug: chromium:752781
Change-Id: If3a2946a62fa38cc668695ed7186b9751a1f356f
Reviewed-on: https://chromium-review.googlesource.com/605894
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47563}