Don't crash if there's no m_image yet.

Reproducible with:
    QBackingStore store( &window );
    store.beginPaint(QRect(0,0,500,500));

All other methods already have null pointer checks.

Change-Id: Ie278a263760900b58cf4a2ef286deb7f35d50cd0
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

src/plugins/platforms/xcb/qxcbbackingstore.cpp

@@ -262,11 +262,14 @@ QXcbBackingStore::~QXcbBackingStore()
 
 QPaintDevice *QXcbBackingStore::paintDevice()
 {
-    return m_image->image();
+    return m_image ? m_image->image() : 0;
 }
 
 void QXcbBackingStore::beginPaint(const QRegion &region)
 {
+    if (!m_image)
+        return;
+
     m_image->preparePaint(region);
 
 #if 0