Fix ConcurrentModificationException in kickPresence

Openfire 3.9.2 is throwing a ConcurrentModificationException when using the "kick" button in the admin panel to remove a user from a group chat room.

java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:859)
	at java.util.ArrayList$Itr.next(ArrayList.java:831)
	at org.jivesoftware.openfire.muc.spi.LocalMUCRoom.kickPresence(LocalMUCRoom.java:2092)
	at org.jivesoftware.openfire.muc.spi.LocalMUCRoom.kickOccupant(LocalMUCRoom.java:2072)
	at org.jivesoftware.openfire.admin.muc_002droom_002doccupants_jsp._jspService(muc_002droom_002doccupants_jsp.java:99)
	at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)

This patch fixes the exception by working off a copy of the list.