Add missing null check to OutputQueue when changing the enabled state of the queue.

Change-Id: I8e74b3d9a247c289c96cd22736bcc4636f222996
Reviewed-on: http://gerrit.dmdirc.com/2839
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>

src/com/dmdirc/parser/irc/outputqueue/OutputQueue.java

@@ -113,8 +113,10 @@ public class OutputQueue {
 
         if (old != queueEnabled && old) {
             synchronized (queueHandlerLock) {
-                queueHandler.interrupt();
-                queueHandler = null;
+                if (queueHandler != null) {
+                    queueHandler.interrupt();
+                    queueHandler = null;
+                }
             }
 
             while (!queue.isEmpty()) {