message: fix async::exec bug
diff: @@ -1065,8 +1065,6 @@ static int process_stream_onRead(swReactor *reactor, swEvent *event)
  SwooleG.main_reactor->del(SwooleG.main_reactor, ps->fd);
  - swString_free(ps->buffer);
  -
  if (ps->buffer->length == 0)
  {
  ZVAL_EMPTY_STRING(&args[0]);
  @@ -1075,6 +1073,7 @@ static int process_stream_onRead(swReactor *reactor, swEvent *event)
  {
  ZVAL_STRINGL(&args[0], ps->buffer->str, ps->buffer->length);
  }
  + swString_free(ps->buffer);
  int status;
  pid_t pid = swWaitpid(ps->pid, &status, WNOHANG);
  