Deny checkpass with empty bind_user
Deny checkpass with empty bind_user

TO fix #179

jh23453/privacyidea,XCage15/privacyidea,woddx/privacyidea,XCage15/privacyidea,jh23453/privacyidea,jalr/privacyidea,privacyidea/privacyidea,XCage15/privacyidea,wheldom01/privacyidea,privacyidea/privacyidea,jh23453/privacyidea,woddx/privacyidea,jh23453/privacyidea,woddx/privacyidea,wheldom01/privacyidea,privacyidea/privacyidea,privacyidea/privacyidea,jalr/privacyidea,jalr/privacyidea,privacyidea/privacyidea,woddx/privacyidea,XCage15/privacyidea,wheldom01/privacyidea,privacyidea/privacyidea,wheldom01/privacyidea,woddx/privacyidea,jh23453/privacyidea,jalr/privacyidea,wheldom01/privacyidea,jalr/privacyidea,wheldom01/privacyidea,woddx/privacyidea,jalr/privacyidea,jh23453/privacyidea,XCage15/privacyidea,XCage15/privacyidea