Fix case when filter returns no queue to work on

[AvnerCohen]

If a Q does not exist, the plugin will fail with a devide by zero exception.
In our use case, we sometimes monitor queues that will be deleted for a while before added back.
This will avoid failing with the non descriptive 'Illegal division by zero' error.

$> /opt/plugins/check_rabbitmq_queue -H $HOSTNAME --port=$PORT -u $username -p $password --filter NON_EXISTING_QUEUE -c 400,400,400
Illegal division by zero at /opt/plugins/check_rabbitmq_queue line 196.