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>

ui/sdl2.c

@@ -521,6 +521,10 @@ static void handle_windowevent(SDL_Event *ev)
 {
     struct sdl2_console *scon = get_scon_from_window(ev->window.windowID);
 
+    if (!scon) {
+        return;
+    }
+
     switch (ev->window.event) {
     case SDL_WINDOWEVENT_RESIZED:
         {