When calling the CWasmEntry in order to call from the interpreter to a
wasm function, the given buffer must hold the arguments, and must also
have enough space to hold the return values. We were missing the second
part, hence we failed when there are no parameters, but a return.

R=ahaas@chromium.org

Bug: chromium:784125
Change-Id: I08d417cae60eea64fda8a72e898dbed9f3e88148
Reviewed-on: https://chromium-review.googlesource.com/771633
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49402}