Fix regression introduced in 37ffc024225fd2ef1016babf3cbbeabce4a32899.
(tty_got_winch): don't read FIFO, just check whether it os empty or not.
It can be called many times without lost of info about SIGWINCH raises.
(tty_flush_winch): read from FIFO to make it empty.
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>