ConnectionExists: fix NTLM check for new connection

When the requested authentication bitmask includes NTLM, we cannot
re-use a connection for another username/password as we then risk
re-using NTLM (connection-based auth).

This has the unfortunate downside that if you include NTLM as a possible
auth, you cannot re-use connections for other usernames/passwords even
if NTLM doesn't end up the auth type used.

Reported-by: Paras S
Patched-by: Paras S
Bug: http://curl.haxx.se/mail/lib-2014-01/0046.html