Fixed crash in tst_qguiapplication with QT_QPA_PLATFORM=minimal

The tst_qguiapplication creates and destroys multiple instances of
QGuiApplication. Since the minimal platform plugin doesn't report any
theme names, the platform_theme never gets set to 0 in init_platform,
and we end up trying to use or at least delete an already deleted
QPlatformTheme.

Change-Id: I1a41d55b0705c5531c019e60a7a96dac144bacb7
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/gui/kernel/qguiapplication.cpp

@@ -1066,6 +1066,7 @@ QGuiApplicationPrivate::~QGuiApplicationPrivate()
     QPixmapCache::clear();
 
     delete platform_theme;
+    platform_theme = 0;
     delete platform_integration;
     platform_integration = 0;
     delete m_gammaTables.load();