Fix error when searching for groups of a non-local user when using LDAP

Exception thrown when searching for userDN based on username 'test@conference.myserver.com/ranga2'
org.jivesoftware.wildfire.user.UserNotFoundException: Username test@conference.myserver.com/ranga2 not found
at org.jivesoftware.wildfire.ldap.LdapManager.findUserDN(LdapManager.java:493)
at org.jivesoftware.wildfire.ldap.LdapManager.findUserDN(LdapManager.java:428)
at org.jivesoftware.wildfire.ldap.LdapGroupProvider.getGroups(LdapGroupProvider.java:260)
at org.jivesoftware.wildfire.group.GroupManager.getGroups(GroupManager.java:283)
at org.jivesoftware.wildfire.roster.Roster.getImplicitRosterItem(Roster.java:231)
at org.jivesoftware.wildfire.roster.Roster.getRosterItem(Roster.java:194)
at org.jivesoftware.wildfire.handler.PresenceUpdateHandler.directedPresenceSent(PresenceUpdateHandler.java:335)
at org.jivesoftware.wildfire.PresenceRouter.handle(PresenceRouter.java:104)
at org.jivesoftware.wildfire.PresenceRouter.route(PresenceRouter.java:61)
at org.jivesoftware.wildfire.PacketRouter.route(PacketRouter.java:73)
at org.jivesoftware.wildfire.net.SocketReader.processPresence(SocketReader.java:445)
at org.jivesoftware.wildfire.net.ClientSocketReader.processPresence(ClientSocketReader.java:56)
at org.jivesoftware.wildfire.net.SocketReader.readStream(SocketReader.java:242)
at org.jivesoftware.wildfire.net.SocketReader.run(SocketReader.java:119)
at java.lang.Thread.run(Unknown Source)