In the test case the module contained a memory which got exported by the
name 'main'. The fuzzer crashed when it tried to cast the memory to a
function to execute it. This CL checks that 'main' is a function before
doint the cast.

R=clemensh@chromium.org

Bug: chromium:763349
Change-Id: I9a21413c8038a7547f8b59057afea2870b15499a
Reviewed-on: https://chromium-review.googlesource.com/659978Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47941}