free file buffer when worker is done

assignment1/partb/550server.cpp

@@ -228,6 +228,7 @@ static void HttpServer_ThrFn(ThreadPool::Task *t)
   while (write_res != 1) {
     write_res = WrappedWrite(hst->work->pipe_fd, (unsigned char *)&check, 1);
   }
+  free(file);
   // 3. and arranges to return (a pointer to) the file content to whoever dispatched the thread to the worker function.
   // TODO: Tell main thread that task is done, main thread needs to close the connection
 }