In chat_UpdateSet(), initialize the input buffer to prevent stale data

from previous timed out commands.

PR:		186530
Submitted by:	Alexander Zagrebin <alexz at visp.ru>
Reviewed by:	brian
MFC after:	1 week

usr.sbin/ppp/chat.c

@@ -154,6 +154,11 @@ chat_UpdateSet(struct fdescriptor *d, fd_set *r, fd_set *w, fd_set *e, int *n)
     else {
       /* c->state = CHAT_EXPECT; */
       c->argptr = &arg_term;
+      /*
+	We have to clear the input buffer, because it contains output
+	from the previous (timed out) command.
+      */
+      c->bufstart = c->bufend;
     }
     c->TimedOut = 0;
   }