subscribe to global stores from script (#2139)

src/compile/Component.ts

@@ -648,7 +648,7 @@ export default class Component {
 				this.add_reference(name.slice(1));
 
 				const variable = this.var_lookup.get(name.slice(1));
-				variable.subscribable = true;
+				if (variable) variable.subscribable = true;
 			} else {
 				this.add_var({
 					name,

test/runtime/samples/store-auto-subscribe-missing-global-script/main.svelte

@@ -0,0 +1,3 @@
+<script>
+	$: $missingGlobal;
+</script>

test/runtime/samples/store-auto-subscribe-missing-global-template/_config.js

@@ -0,0 +1,3 @@
+export default {
+	error: 'missingGlobal is not defined'
+};