ppp: fix a buffer overrun in the ms chap code

https://dev.openwrt.org/ticket/17296

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41882 3c298f89-4303-0410-b956-a3cf2f4a3e73