ppp: reject claimed-as-LCP but actually malformed packets

Since 'ppp_async_encode()' assumes valid LCP packets (with code
from 1 to 7 inclusive), add 'ppp_check_packet()' to ensure that
LCP packet has an actual body beyond PPP_LCP header bytes, and
reject claimed-as-LCP but actually malformed data otherwise.

Reported-by:  <syzbot+ec0723ba9605678b14bf@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=ec0723ba9605678b14bf
Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>