[wasm] [fuzzer] Fix segfault

Even though we were generating additional arguments with default value
in the case that the caller was not providing enough, we then passed
the original pointer, leading to potential out-of-bounds accesses.

R=ahaas@chromium.org

Bug: chromium:763294,chromium:763297
Change-Id: Id18622d0d40e0408e26a5fc6f97494b5f9e18d17
Reviewed-on: https://chromium-review.googlesource.com/657699Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47930}

test/common/wasm/wasm-module-runner.cc

@@ -88,7 +88,8 @@ bool InterpretWasmModuleForTesting(Isolate* isolate,
   WasmInterpreter::HeapObjectsScope heap_objects_scope(interpreter, instance);
   WasmInterpreter::Thread* thread = interpreter->GetThread(0);
   thread->Reset();
-  thread->InitFrame(&(instance->module()->functions[function_index]), args);
+  thread->InitFrame(&instance->module()->functions[function_index],
+                    arguments.get());
   WasmInterpreter::State interpreter_result = thread->Run(kMaxNumSteps);
 
   return interpreter_result != WasmInterpreter::PAUSED;