coverity#704762 Dereference after null check

Change-Id: I28555dcf30eb3a6f71b3808025771294a088aaa8

sd/source/ui/slideshow/slideshowimpl.cxx

@@ -673,6 +673,7 @@ void SAL_CALL SlideshowImpl::disposing()
     if( mpViewShell )
     {
         mpViewShell->SetActiveWindow(mpOldActiveWindow);
+        if (mpShowWindow)
             mpShowWindow->SetViewShell( NULL );
     }