Fix crash when C++ QJSValue parameterized signal interacts with JS

When converting the parameters of a C++ signal to JS values to provide
to a signal handler written in JS, the conversion of a QJSValue to a
QV4::Value* may yield a null pointer in case of a default constructed
QJSValue for example. This is a regression from commit
aa869cbb06 and we must check for this.

Task-number: QTBUG-58133
Change-Id: I528b606b2851dfb3072e54902bd8843d31571a55
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/qml/qml/qqmlboundsignal.cpp

@@ -199,7 +199,10 @@ void QQmlBoundSignalExpression::evaluate(void **a)
         //    for several cases (such as QVariant type and QObject-derived types)
         //args[ii] = engine->metaTypeToJS(type, a[ii + 1]);
         if (type == qMetaTypeId<QJSValue>()) {
-            callData->args[ii] = *QJSValuePrivate::getValue(reinterpret_cast<QJSValue *>(a[ii + 1]));
+            if (QV4::Value *v4Value = QJSValuePrivate::getValue(reinterpret_cast<QJSValue *>(a[ii + 1])))
+                callData->args[ii] = *v4Value;
+            else
+                callData->args[ii] = QV4::Encode::undefined();
         } else if (type == QMetaType::QVariant) {
             callData->args[ii] = scope.engine->fromVariant(*((QVariant *)a[ii + 1]));
         } else if (type == QMetaType::Int) {

tests/auto/qml/qqmlecmascript/data/signalParameterTypes.qml

@@ -15,4 +15,6 @@ MyQmlObject
     onMySignal: { intProperty = a; realProperty = b; colorProperty = c; variantProperty = d; enumProperty = e; qtEnumProperty = f; }
 
     onBasicSignal: root.mySignal(10, 19.2, Qt.rgba(1, 1, 0, 1), Qt.rgba(1, 0, 1, 1), MyQmlObject.EnumValue3, Qt.LeftButton)
+
+    onQjsValueEmittingSignal: {}
 }

tests/auto/qml/qqmlecmascript/testtypes.h

@@ -244,6 +244,7 @@ signals:
     void signalWithGlobalName(int parseInt);
     void intChanged();
     void qjsvalueChanged();
+    void qjsValueEmittingSignal(QJSValue value);
 
 public slots:
     void deleteMe() { delete this; }

tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp

@@ -1410,6 +1410,7 @@ void tst_qqmlecmascript::signalParameterTypes()
     QVERIFY(object != 0);
 
     emit object->basicSignal();
+    emit object->qjsValueEmittingSignal(QJSValue());
 
     QCOMPARE(object->property("intProperty").toInt(), 10);
     QCOMPARE(object->property("realProperty").toReal(), 19.2);