Fix is_my_tty() buffer overrun

Fix is_my_tty() buffer overrun

*  libmisc/utmp.c (is_my_tty): Declare the parameter as a char array,
   not char *, as it is not necessarily null-terminated.
   Avoid a read overrun when reading 'tty', which comes from
   'ut_utname'.

Reported-by: Paul Eggert <eggert@cs.ucla.edu>
Co-developed-by: Paul Eggert <eggert@cs.ucla.edu>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>