stty: ensure arbitrary data is not displayed

stty: ensure arbitrary data is not displayed

* src/stty.c (main): Use static structures to ensure
they're initialized (to zero), so that random data is
not displayed, or compared resulting in a inaccurate
failure reported to users.  This was seen on musl libc
where some parts of the termios c_cc array were
not initialized by tcgetattr().
Reported by Bruno Haible.