Simple component-exists check to avoid throwing exceptions

[banjerluke]

My Svelte-in-Blaze app was sometimes throwing errors in this part of SvelteComponent.js:

Template.SvelteComponent.onDestroyed(function onDestroyed() {
  this.component.$destroy();
});

My guess is that the template was getting destroyed before it had a chance to be rendered. I don't know why, but adding a check to see if this.component exists before calling .$destroy() seemed like an easy and pragmatic solution. Seems to be working well now with no side effects that I can detect.

[banjerluke]

Side note to the maintainer(s): thank you so much for your work in making Svelte and Blaze work together. My app is pretty large now, and the thought of migrating the whole thing over to Svelte is basically unthinkable. But thanks to your packages, I was able to migrate just a few templates that need to be super-high-performance, and was able to throw away a ton of ugly raw-DOM kludges I was using with Blaze to get the necessary performance and ended up with something many times faster than what I started with. Kudos! 😃