Avoid stealing data from an input program that uses the tty

at startup, like sudo.

os.c

@@ -114,6 +114,8 @@ static int check_poll(int fd, int tty)
 {
  struct pollfd poller[2] = { { fd, POLLIN, 0 }, { tty, POLLIN, 0 } };
  int timeout = (waiting_for_data && !(scanning_eof && follow_mode == FOLLOW_NAME)) ? -1 : waiting_for_data_delay;
+ if (!any_data)
+ return (0);
  poll(poller, 2, timeout);
 #if LESSTEST
  if (ttyin_name == NULL) /* Check for ^X only on a real tty. */
@@ -136,7 +138,7 @@ static int check_poll(int fd, int tty)
  * to allow a program piping data into less to have temporary
  * access to the tty (like sudo asking for a password).
  */
- if (any_data && (poller[0].revents & (POLLIN|POLLHUP|POLLERR)) == 0)
+ if ((poller[0].revents & (POLLIN|POLLHUP|POLLERR)) == 0)
  /* No data available; let caller take action, then try again. */
  return (READ_AGAIN);
  /* There is data (or HUP/ERR) available. Safe to call read() without blocking. */