If no screens, attempting to create a window results in clean exit

Asserting is only relevant for debug builds, and there were a couple
of other places that a segfault would typically occur before the assert
had a chance.

Change-Id: I1abc82eb3ecfa91050117fab1525f4cbd82ff486
Task-number: QTBUG-37876
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/gui/image/qnativeimage.cpp

@@ -64,6 +64,8 @@ QNativeImage::~QNativeImage()
 
 QImage::Format QNativeImage::systemFormat()
 {
+    if (!QGuiApplication::primaryScreen())
+        return QImage::Format_Invalid;
     return QGuiApplication::primaryScreen()->handle()->format();
 }
 

src/gui/kernel/qwindow.cpp

@@ -159,10 +159,6 @@ QWindow::QWindow(QScreen *targetScreen)
     d->screen = targetScreen;
     if (!d->screen)
         d->screen = QGuiApplication::primaryScreen();
-
-    //if your applications aborts here, then chances are your creating a QWindow before the
-    //screen list is populated.
-    Q_ASSERT(d->screen);
     d->init();
 }
 
@@ -232,6 +228,13 @@ QWindow::~QWindow()
 void QWindowPrivate::init()
 {
     Q_Q(QWindow);
+
+    // If your application aborts here, you are probably creating a QWindow
+    // before the screen list is populated.
+    if (!screen) {
+        qFatal("Cannot create window: no screens available");
+        exit(1);
+    }
     QObject::connect(screen, SIGNAL(destroyed(QObject*)), q, SLOT(screenDestroyed(QObject*)));
     QGuiApplicationPrivate::window_list.prepend(q);
 }

src/widgets/util/qcolormap.cpp

@@ -66,7 +66,12 @@ static QColormapPrivate *screenMap = 0;
 void QColormap::initialize()
 {
     screenMap = new QColormapPrivate;
-
+    if (!QGuiApplication::primaryScreen()) {
+        qWarning("no screens available, assuming 24-bit color");
+        screenMap->depth = 24;
+        screenMap->mode = QColormap::Direct;
+        return;
+    }
     screenMap->depth = QGuiApplication::primaryScreen()->depth();
     if (screenMap->depth < 8) {
         screenMap->mode = QColormap::Indexed;