add test and fix code

src/runtime/store/index.ts

@@ -98,9 +98,9 @@ export function writable<T>(value?: T, start: StartStopNotifier<T> = noop): Writ
 
 		return () => {
 			subscribers.delete(subscriber);
-			if (subscribers.size === 0) {
+			if (subscribers.size === 0 && stop) {
 				stop();
-				stop = noop;
+				stop = null;
 			}
 		};
 	}

test/store/index.ts

@@ -88,6 +88,14 @@ describe('store', () => {
 
 			unsubscribe();
 		});
+
+		it('no error even if unsubscribe calls twice', () => {
+			let num = 0;
+			const store = writable(num, set => set(num += 1));
+			const unsubscribe = store.subscribe(() => { });
+			unsubscribe();
+			assert.doesNotThrow(() => unsubscribe());
+		});
 	});
 
 	describe('readable', () => {