linuxfb: Do not crash with GL windows

We will show an error message saying no platform context is available
but the crash (due to GL windows having a null backingstore) is not
desirable.

Change-Id: Iba3a61bfc4eeeb89b4a0017a58c87a7dbd0895e7
Reviewed-by: Andy Nichols <andy.nichols@digia.com>

src/platformsupport/fbconvenience/qfbscreen.cpp

@@ -258,8 +258,9 @@ QRegion QFbScreen::doRedraw()
                     QRect windowRect = mWindowStack[layerIndex]->geometry().translated(-screenOffset);
                     QRect windowIntersect = rect.translated(-windowRect.left(),
                                                             -windowRect.top());
-                    mCompositePainter->drawImage(rect, mWindowStack[layerIndex]->backingStore()->image(),
-                                                windowIntersect);
+                    QFbBackingStore *backingStore = mWindowStack[layerIndex]->backingStore();
+                    if (backingStore)
+                        mCompositePainter->drawImage(rect, backingStore->image(), windowIntersect);
                     if (firstLayer) {
                         firstLayer = false;
                     }