[wasm][fuzzer] Second instantiation cannot fail

A minor fix to the {InterpretAndExecuteModule} function: We instantiate
the module twice. If the first instantiation worked, then also the
second instantiation must succeed.
Plus minor drive-by cleanup.

R=ahaas@chromium.org

Bug: chromium:1113681
Change-Id: Ib897cb1907152cdd9b0ed2b513a6c8217a3f400c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2349288
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69352}

test/fuzzer/wasm-fuzzer-common.cc

@@ -33,17 +33,15 @@ void InterpretAndExecuteModule(i::Isolate* isolate,
   if (module_object->module()->start_function_index >= 0) return;
 
   HandleScope handle_scope(isolate);  // Avoid leaking handles.
-  MaybeHandle<WasmInstanceObject> maybe_instance;
   Handle<WasmInstanceObject> instance;
 
   // Try to instantiate, return if it fails.
   {
     ErrorThrower thrower(isolate, "WebAssembly Instantiation");
-    maybe_instance = isolate->wasm_engine()->SyncInstantiate(
-        isolate, &thrower, module_object,
-        Handle<JSReceiver>::null(),     // imports
-        MaybeHandle<JSArrayBuffer>());  // memory
-    if (!maybe_instance.ToHandle(&instance)) {
+    if (!isolate->wasm_engine()
+             ->SyncInstantiate(isolate, &thrower, module_object, {},
+                               {})  // no imports & memory
+             .ToHandle(&instance)) {
       isolate->clear_pending_exception();
       thrower.Reset();  // Ignore errors.
       return;
@@ -76,16 +74,12 @@ void InterpretAndExecuteModule(i::Isolate* isolate,
 
   // Try to instantiate and execute the module_object.
   {
-    ErrorThrower thrower(isolate, "InterpretAndExecuteModule");
-    maybe_instance = isolate->wasm_engine()->SyncInstantiate(
-        isolate, &thrower, module_object,
-        Handle<JSReceiver>::null(),     // imports
-        MaybeHandle<JSArrayBuffer>());  // memory
-    if (!maybe_instance.ToHandle(&instance)) {
-      isolate->clear_pending_exception();
-      thrower.Reset();  // Ignore errors.
-      return;
-    }
+    ErrorThrower thrower(isolate, "Second Instantiation");
+    // We instantiated before, so the second instantiation must also succeed:
+    CHECK(isolate->wasm_engine()
+              ->SyncInstantiate(isolate, &thrower, module_object, {},
+                                {})  // no imports & memory
+              .ToHandle(&instance));
   }
 
   int32_t result_compiled = testing::CallWasmFunctionForTesting(