streams: update ongoing promise in async iterator return() method

Author: MattiasBuelens

lib/internal/webstreams/readablestream.js

@@ -545,12 +545,14 @@ class ReadableStream {
       },
 
       return(error) {
-        return state.current ?
+        started = true;
+        state.current = state.current !== undefined ?
           PromisePrototypeThen(
             state.current,
             () => returnSteps(error),
             () => returnSteps(error)) :
           returnSteps(error);
+        return state.current;
       },
 
       [SymbolAsyncIterator]() { return this; },