iOS: ensure we return a non-null clipboard

QPlatformIntegration::clipboard() is not allowed to return
 null. This will lead to a crash as soon as someone tries
 to access QClipboard.

Change-Id: I3c1e7cdc5103fe4424f9739a09f599d6c0af40b2
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

src/plugins/platforms/ios/qiosintegration.mm

@@ -227,7 +227,7 @@ QPlatformClipboard *QIOSIntegration::clipboard() const
 #ifndef Q_OS_TVOS
     return m_clipboard;
 #else
-    return 0;
+    return QPlatformIntegration::clipboard();
 #endif
 }
 #endif