Add model null pointer check (solves issue #595 and https://github.com/laanwj/bitcoin-qt/issues/29)

Missed this one before, which can be triggered in race condition if window event arrives before model is set

src/qt/bitcoingui.cpp

@@ -480,7 +480,7 @@ void BitcoinGUI::changeEvent(QEvent *e)
 #ifndef Q_WS_MAC // Ignored on Mac
     if(e->type() == QEvent::WindowStateChange)
     {
-        if (clientModel->getOptionsModel()->getMinimizeToTray())
+        if(clientModel && clientModel->getOptionsModel()->getMinimizeToTray())
         {
             if(isMinimized())
             {