pppd: Don't crash if crypt() returns NULL

It is possible for crypt() to return NULL under some circumstances,
so we need to check the return value before passing it to strcmp().
If we do get NULL from crypt(), treat it as an authentication failure.

Reported-by: Paul Wouters <pwouters@redhat.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>