sdl: Move use of surface pointer below check for whether it is NULL

sdl: Move use of surface pointer below check for whether it is NULL

In commit 2ab858c6c38ee1 we added a use of the 'surf' variable
in sdl2_2d_update() that was unfortunately placed above the
early-exit-if-NULL check. Move it to where it ought to be.

Fixes: Coverity CID 1390598
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180515185814.1374-1-peter.maydell@linaro.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>