Fix crash: Handle invalid base64 in NTLM challenge.

Fix crash: Handle invalid base64 in NTLM challenge.

Some servers, for instance the MS Exchange servers deployed by the
US-American National Aeronautics and Space Administration (NASA),
aborted the NTLM protocol exchange after receiving the initial request.

Fetchmail did not detect that there was an error message, rather than
NTLM protocol exchange, and caught a segmentation fault while reading
from a bad location.

Detect base64 decoding errors, and return PS_AUTHFAIL in this case.

Reported by J[ames] Porter Clark.