WasmModule::Instantiate can return an empty handle if it is not possible
to instantiate the module. With this change the wasm-module-runner does
not assume anymore that WasmModule::Instantiate produces a valid handle.

BUG=chromium:648078
R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2365493003
Cr-Commit-Position: refs/heads/master@{#39629}