Fix the code to handle the Password Authentication Protocol

Fix the code to handle the Password Authentication Protocol
Authenticate-Ack and Authenticate-Nak packets to do a length check based
on "msg_len", which we set in that case, rather than "passwd_len", which
we don't.  Thanks and a tip of the Hatlo Hat to some non-GCC compiler -
I forget which one - that noticed that we were using a variable which
we hadn't necessarily set.