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>

pppd/options.c

@@ -1357,6 +1357,7 @@ getword(f, word, newlinep, filename)
 
 	c = getc(f);
     }
+    word[MAXWORDLEN-1] = 0;	/* make sure word is null-terminated */
 
     /*
      * End of the word: check for errors.