fix trying to update unmounted input

[AlbertLucianto]

Should fix #3763

Referring to the issue, when an InlineComponent is rendered inside a slot, it is always instantiated and subscribes to the store without being mounted. As a result, when the store is updated it fails because it's trying to set the value to an unmounted input element.

I tried conditionally instantiating the default slot if custom slot is not given, but it breaks on nested-slot test case. So, I think the safest and simplest solution is to check whether the DOM element exists before updating the value.

It's my first time contribution, and really appreciate for your review! 😄

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Tests

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[Rich-Harris]

Thank you for the PR! I think this is actually masking a deeper bug — in the original repro, this code gets generated for the Outer component:

function create_fragment(ctx) {
	var current;

	const default_slot_template = ctx.$$slots.default;
	const default_slot = create_slot(default_slot_template, ctx, null);

	var inner = new Inner({});

	return {...};
}

In other words, the Inner component gets instantiated no matter whether there's a default slot. Having been instantiated, it subscribes to the store, which triggers an invalidation that causes an update, but the update fails because the component was never 'created'.

So I think the solution here is to prevent that from happening, which may be slightly finicky — we'll see. I'll note this on #3763 and close this PR.