pppd: Make sure word read from options file is null-terminated

If a word read from an options file was longer than MAXWORDLEN,
we could pass it to option_error() without null termination,
which could have lead to an out-of-bounds access in vslprintf.
Make sure word[] is null terminated in all cases.

Reported-by: Florian Kohnhäuser <florian@kohnhaeuser.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>