sdl2: add assert to make coverity happy

There is a loop a few lines up counting consoles and setting
sdl2_num_outputs accordingly, so con ptr can't be NULL there.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170621122234.12751-1-kraxel@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
