An invalid I32V value as index could be used to get a valid
WasmFunction.

R=clemensh@chromium.org

Bug: chromium:735887
Change-Id: I5fbfa01fc3300d86a4a2ba9bcbb86fc02f231ef9
Reviewed-on: https://chromium-review.googlesource.com/561536Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46504}