In the original CL I moved an error check backwards, unfortunately
behind a vector lookup which should not happen when there is an error.
Now I also move the vector lookup backwards.

Original message:
We stored the size of a function body before we check that
these values are valid. This caused a failing DCHECK in the constructor
of WireBytesRef which checked for integer overflows. With this CL we
check the size of the function body before we create the WireBytesRef.

R=clemensh@chromium.org

Bug: chromium:738097
Change-Id: Ie65b3cfcbcd6bdb3f04b0760673d9c7b7a0d1057
Reviewed-on: https://chromium-review.googlesource.com/561519Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46453}