Ensure client modes are non-null.

Fixes #82.

irc/src/com/dmdirc/parser/irc/IRCClientInfo.java

 import java.util.List;
 import java.util.Map;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 /**
  * Contains information about known users.
  *
     /** Known host of client. */
     private String host = "";
     /** Known user modes of client. */
-    private String modes;
+    private String modes = "";
     /** Known Away Reason of client. */
     private String awayReason = "";
     /** Known Account name of client. */
      * @param newMode new string containing boolean channel modes.
      */
     public void setUserMode(final String newMode) {
-        modes = newMode;
+        modes = checkNotNull(newMode);
     }
 
     /**