Instead of using the size of the whole code object, just use the size
of the instructions, because only there faults can happen.

R=eholk@chromium.org

Bug: v8:5277
Change-Id: Ia5768891ec3c1ee5ad8affc9486e044d79e23146
Reviewed-on: https://chromium-review.googlesource.com/712536Reviewed-by: Eric Holk <eholk@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48464}