[wasm][fuzzer] Do not execute modules with start function

In the WebAssembly fuzzers we detect infinite loops with the
interpreter: if the interpreter does not finish after a finite number
of steps, we do not execute the compiled code. However, we cannot
redirect the start function to the interpreter in the fuzzer, and
therefore we cannot detect infinite loops in the start function. With
this CL we avoid the problem completely by not instantiating a module
in the fuzzer which has a start function. Note that the module still
gets compiled.

R=clemensh@chromium.org

Bug: chromium:858914
Change-Id: Icbbe9a003544918d5267cdd1d9405b21bb681133
Reviewed-on: https://chromium-review.googlesource.com/1126766
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54246}

test/fuzzer/wasm-fuzzer-common.cc

@@ -67,6 +67,10 @@ int FuzzWasmSection(SectionCode section, const uint8_t* data, size_t size) {
 
 void InterpretAndExecuteModule(i::Isolate* isolate,
                                Handle<WasmModuleObject> module_object) {
+  // We do not instantiate the module if there is a start function, because a
+  // start function can contain an infinite loop which we cannot handle.
+  if (module_object->module()->start_function_index >= 0) return;
+
   ErrorThrower thrower(isolate, "WebAssembly Instantiation");
   MaybeHandle<WasmInstanceObject> maybe_instance;
   Handle<WasmInstanceObject> instance;