wined3d: Only read "*rect_count" when "rects" is non-NULL in wined3d_device_context_get_scissor_rects() (Valgrind).

When "rects" is NULL, "*rect_count" is potentially uninitialised. That's
fairly benign because the resulting "count" value is inconsequential in that
case, but it's also unnecessary, and easy to avoid.
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>