sdl2: fix crash in handle_windowevent() when restoring the screen size

The Ctrl-Alt-u keyboard shortcut restores the screen to its original
size. In the SDL2 UI this is done by destroying the window and
creating a new one. The old window emits SDL_WINDOWEVENT_HIDDEN when
it's destroyed, but trying to call SDL_GetWindowFromID() from that
event's window ID returns a null pointer. handle_windowevent() assumes
that the pointer is never null so it results in a crash.

Cc: qemu-stable@nongnu.org
Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
