Fix string comparison for Basic auth

Perhaps we should be using g_ascii_strcasecmp(). But this is sometimes
NULL, for NTLM auth, and there's no g_ascii_strcasecmp0().

Perhaps we should fix that so it consistently returns a non-NULL string,
rather than using NULL for NTLM...

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>