gdi32: Return failure when GetPixel is not supported.

dlls/gdi32/driver.c

@@ -402,7 +402,7 @@ static UINT nulldrv_GetOutlineTextMetrics( PHYSDEV dev, UINT size, LPOUTLINETEXT
 
 static COLORREF nulldrv_GetPixel( PHYSDEV dev, INT x, INT y )
 {
-    return 0;
+    return CLR_INVALID;
 }
 
 static INT nulldrv_GetPixelFormat( PHYSDEV dev )

dlls/gdi32/tests/dc.c

@@ -873,6 +873,9 @@ static void test_printer_dc(void)
     ok( BitBlt( memdc, 10, 10, 20, 20, display_memdc, 0, 0, SRCCOPY ), "BitBlt failed\n" );
     ok( BitBlt( display_memdc, 10, 10, 20, 20, memdc, 0, 0, SRCCOPY ), "BitBlt failed\n" );
 
+    ret = GetPixel( hdc, 0, 0 );
+    ok( ret == CLR_INVALID, "wrong pixel value %x\n", ret );
+
     DeleteDC( memdc );
     DeleteDC( display_memdc );
     DeleteDC( hdc );