sdl2: add checking for NULL

Sometimes SDL2 console can't be retrieved on events and scon variable
becomes NULL. This patch prevents processing of the events when the console
is not available.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
Signed-off-by: Maria Klimushenkova <maria.klimushenkova@ispras.ru>
Message-id: 20180626064729.18070.47600.stgit@pasha-VirtualBox
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>