coverity#1242365: Unchecked return value

Change-Id: Iddb8b1bad29bf372f9485514e2cde195237e3cc3
diff --git a/vcl/source/window/window.cxx b/vcl/source/window/window.cxx
index 27beb71..8b52d5c 100644
--- a/vcl/source/window/window.cxx
+++ b/vcl/source/window/window.cxx
@@ -1292,8 +1292,11 @@ SalGraphics* Window::ImplGetFrameGraphics() const
    }
    else
    {
        OutputDevice *pFrameWinOutDev = mpWindowImpl->mpFrameWindow;
        pFrameWinOutDev->AcquireGraphics();
        OutputDevice* pFrameWinOutDev = mpWindowImpl->mpFrameWindow;
        if ( ! pFrameWinOutDev->AcquireGraphics() )
        {
            return NULL;
        }
    }
    mpWindowImpl->mpFrameWindow->mpGraphics->ResetClipRegion();
    return mpWindowImpl->mpFrameWindow->mpGraphics;