x11_rawfb demo attempts two double-frees if XShm is not available

In `demo/x11_rawfb/nuklear_xlib.h`, if `XShmQueryExtension` fails and sets `xlib.fallback` to true, then `nk_xlib_shutdown` will end up being called twice from `nk_handle_event` which will try to double free two different things (found using AddressSanitizer):

```c
if (xlib.fallback) {
free(xlib.ximg->data);
XDestroyImage(xlib.ximg);
```

I'm not sure why the nk_handle_event (and nk_xlib_shutdown function) are called twice, but a quick fix would be to set these two variables to NULL after freeing them, and then check for null before calling the free in the first place.

Start in demo/x11_rawfb/nuklear_xlib.h by tracing nk_handle_event and nk_xlib_shutdown when XShmQueryExtension fails and xlib.fallback is true. Reproduce the path with AddressSanitizer and verify that repeated shutdown does not double-free xlib.ximg->data or xlib.ximg.

Written by the indexing model from the issue text.