Crash in _emscripten_set_wheel_callback_on_thread

When my WASM code calls SDL_DestroyWindow, it proceeds to unregister various callbacks. As part of this it calls _emscripten_set_wheel_callback_on_thread, which throws an exception `null is not an object (evaluating 'target.onwheel')`, because `target` is null, because the canvas has already been removed from the DOM.

SDL_DestroyWindow is ultimately called from componentDidUnmount by Reactjs. The canvas is there at the time of SDL_DestroyWindow, but _emscripten_set_wheel_callback_on_thread is called asynchronously on the next tick (because of the _on_thread) and by that time the canvas is gone.

The offending line is:

https://github.com/emscripten-core/emscripten/blob/77da62969eb0b5efe4ce4a0c9ce67c45b9e32638/src/library_html5.js#L700

I think this line should check that target isn't null and do nothing if it is.