Fix bug #737.

The problem was the the splash window was displayed
when the FFmpeg warning dialog appeared.  Just needed
to hide it before deleting the window since the actual
deletion is defered to some time later.

src/AudacityApp.cpp

@@ -1246,6 +1246,7 @@ bool AudacityApp::OnInit()
       pWnd->Show( true );
    }
 
+   temporarywindow->Show( false );
    delete temporarywindow;
 
    if( project->mShowSplashScreen )