Fix crash when modifying list model in worker thread

If we call get() on a model in a worker thread, we may end up creating a
ModelNodeMetaObject (aka cacheObject). Subsequent mutation of properties
may make us end up in emitDirectNotifies(). However since we can't have
bindings in there, we should shortcut/suppress the notify emission, which
we can do by checking ddata->context via qmlEngine(). The previous code
crashed when qmlEngine() return a null pointer but
QQmlEnginePrivate::get(const QQmlEngine *) would attempt to dereference
the parameter.

Started-by: Slava Monich<slava.monich@jolla.com>
Change-Id: I880619c686436c053692faafa5dba2c96c2ace96
Reviewed-by: Robin Burchell <robin.burchell@crimson.no>
Reviewed-by: Slava Monich <slava.monich@jolla.com>

src/qml/types/qqmllistmodel.cpp

@@ -1326,8 +1326,8 @@ void ModelNodeMetaObject::emitDirectNotifies(const int *changedRoles, int roleCo
     QQmlData *ddata = QQmlData::get(object(), /*create*/false);
     if (!ddata)
         return;
-    QQmlEnginePrivate *ep = QQmlEnginePrivate::get(qmlEngine(m_model));
-    if (!ep)
+    // There's nothing to emit if we're a list model in a worker thread.
+    if (!qmlEngine(m_model))
         return;
     for (int i = 0; i < roleCount; ++i) {
         const int changedRole = changedRoles[i];

tests/auto/qml/qqmllistmodelworkerscript/tst_qqmllistmodelworkerscript.cpp

@@ -204,6 +204,7 @@ void tst_qqmllistmodelworkerscript::dynamic_data()
         QTest::newRow("get4") << "{append({'foo':123});get(0).foo}" << 123 << "" << dr;
         QTest::newRow("get-modify1") << "{append({'foo':123,'bar':456});get(0).foo = 333;get(0).foo}" << 333 << "" << dr;
         QTest::newRow("get-modify2") << "{append({'z':1});append({'foo':123,'bar':456});get(1).bar = 999;get(1).bar}" << 999 << "" << dr;
+        QTest::newRow("get-set") << "{append({'foo':123});get(0).foo;setProperty(0, 'foo', 999);get(0).foo}" << 999 << "" << dr;
 
         QTest::newRow("append1") << "{append({'foo':123});count}" << 1 << "" << dr;
         QTest::newRow("append2") << "{append({'foo':123,'bar':456});count}" << 1 << "" << dr;