Update merge.ts

Author: trxcllnt

src/asynciterable/merge.ts

@@ -42,8 +42,10 @@ export class MergeAsyncIterable<T> extends AsyncIterableX<T> {
         nexts[index] = <Promise<MergeResult<IteratorResult<T>>>>NEVER_PROMISE;
         active--;
       } else {
+        const iterator$ = iterators[index];
+        nexts[index] = <Promise<MergeResult<IteratorResult<T>>>>NEVER_PROMISE;
         yield value$;
-        nexts[index] = wrapPromiseWithIndex(iterators[index].next(), index);
+        nexts[index] = wrapPromiseWithIndex(iterator$.next(), index);
       }
     }
   }