Handle undefined from JSON.stringify

mini_racer uses V8's JSON.stringify to serialize results but it
returns undefined instead of a string for certain inputs, like WASM
string refs.

When that undefined value is then passed to Ruby's JSON parser, it
raises an exception (using rb_raise) that longjmps out of a lexical
scope where we hold the v8::Locker, without properly unlocking it
again.

Fixes: #320

ext/mini_racer_extension/mini_racer_extension.cc

@@ -429,14 +429,17 @@ static void prepare_result(MaybeLocal<Value> v8res,
                 Local<Object> object = local_value->ToObject(context).ToLocalChecked();
                 const unsigned argc = 1;
                 Local<Value> argv[argc] = { object };
-                MaybeLocal<Value> json = stringify->Call(context, JSON, argc, argv);
+                MaybeLocal<Value> maybe_json = stringify->Call(context, JSON, argc, argv);
+                Local<Value> json;
 
-                if (json.IsEmpty()) {
+                if (!maybe_json.ToLocal(&json)) {
                     evalRes.executed = false;
                 } else {
-                    evalRes.json = true;
+                    // JSON.stringify() returns undefined for inputs that
+                    // are exotic objects, like WASM function or string refs
+                    evalRes.json = !json->IsUndefined();
                     Persistent<Value>* persistent = new Persistent<Value>();
-                    persistent->Reset(isolate, json.ToLocalChecked());
+                    persistent->Reset(isolate, json);
                     evalRes.value = persistent;
                 }
             }

test/mini_racer_test.rb

@@ -1049,6 +1049,15 @@ def test_deep_object_js
     end
   end
 
+  def test_wasm_ref
+    context = MiniRacer::Context.new
+    assert_nil context.eval("
+      var b = [0,97,115,109,1,0,0,0,1,26,5,80,0,95,0,80,0,95,1,127,0,96,0,1,110,96,1,100,2,1,111,96,0,1,100,3,3,4,3,3,2,4,7,26,2,12,99,114,101,97,116,101,83,116,114,117,99,116,0,1,7,114,101,102,70,117,110,99,0,2,9,5,1,3,0,1,0,10,23,3,8,0,32,0,20,2,251,27,11,7,0,65,12,251,0,1,11,4,0,210,0,11,0,44,4,110,97,109,101,1,37,3,0,11,101,120,112,111,114,116,101,100,65,110,121,1,12,99,114,101,97,116,101,83,116,114,117,99,116,2,7,114,101,102,70,117,110,99]
+      var o = new WebAssembly.Instance(new WebAssembly.Module(new Uint8Array(b))).exports
+      o.refFunc()(o.createStruct) // exotic object
+    ")
+  end
+
   def test_stackdepth_bounds
     assert_raises(ArgumentError) do
       MiniRacer::Context.new(marshal_stack_depth: -2)