Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56246
Fix NullPointerException in MemoryRealm when authenticating an unknown user.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1576770 13f79535-47bb-0310-9956-ffa450edef68