When the mode parameter is MWT_IDENTITY, the xform parameter can be NULL because it should be
ignored and use the identity matrix.

Fix Tally crashing when exporting a JPEG image.
Signed-off-by: Zhiyi Zhang <zzhang@codeweavers.com>
Signed-off-by: Huw Davies <huw@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>