Handle rhi init failures better in rhi backingstore flush

One may very well force a 3D API not functional. In this case there will
be no QRhi. A bunch of warnings will be printed (which is good), but
then flush() should not do anything, and that was not handled before.

Change-Id: I82139070311152c959d39a553842f4462d8e7811
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
(cherry picked from commit 108b1014415a491cb0abb58c04a28a5c691085a1)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/painting/qbackingstoredefaultcompositor.cpp

@@ -455,6 +455,9 @@ QPlatformBackingStore::FlushResult QBackingStoreDefaultCompositor::flush(QPlatfo
                                                                          QPlatformTextureList *textures,
                                                                          bool translucentBackground)
 {
+    if (!rhi)
+        return QPlatformBackingStore::FlushFailed;
+
     Q_ASSERT(textures); // may be empty if there are no render-to-texture widgets at all, but null it cannot be
 
     if (!m_rhi) {