Explicitly remove transactions from queue on shutdown (#1376)

Signed-off-by: Chris Jackson <chris@cd-jackson.com>

Author: cdjackson

com.zsmartsystems.zigbee/src/main/java/com/zsmartsystems/zigbee/transaction/ZigBeeTransactionQueue.java

@@ -132,7 +132,11 @@ protected void shutdown() {
         isShutdown = true;
 
         // Cancel any queued transactions
-        for (ZigBeeTransaction transaction : queue) {
+        while (!queue.isEmpty()) {
+            ZigBeeTransaction transaction = queue.poll();
+            if (transaction == null) {
+                break;
+            }
             transaction.cancel();
         }