Fix current component not being cleared after component update (#4909)

src/runtime/internal/scheduler.ts

@@ -45,6 +45,7 @@ export function flush() {
 			set_current_component(component);
 			update(component.$$);
 		}
+		set_current_component(null);
 
 		dirty_components.length = 0;
 

test/runtime/samples/onmount-get-current-component/_config.js

@@ -0,0 +1,4 @@
+export default {
+	skip_if_ssr: true,
+	html: `1`,
+};

test/runtime/samples/onmount-get-current-component/main.svelte

@@ -0,0 +1,16 @@
+<script>
+	import { onMount } from 'svelte';
+	import { get_current_component } from 'svelte/internal';
+
+	let gotException = 0;
+
+	onMount(() => {
+		try {
+			get_current_component();
+		} catch (error) {
+			gotException++;
+		}
+	});
+</script>
+
+{gotException}