virtualSelectorCacheBrokerPlugin causes a NullPointerException if the kahadb store is not using the default location

Issue

If the KahaDB store has been configured to use a location other than the default data directory and the virtualSelectorCacheBrokerPlugin is in use the following FileNotFoundException occurs every 10 mins:

2017-08-24 17:59:15,593 | ERROR | chePersistThread | SubQueueSelectorCacheBroker      | 162 - org.apache.activemq.activemq-osgi - 5.11.0.redhat-630187 | Unable to access file[/Users/jsherman/Development/tools/jboss-amq/jboss-a-mq-6.3.0.redhat-187/data/amq/selectorcache.data]
java.io.FileNotFoundException: /Users/jsherman/Development/tools/jboss-amq/jboss-a-mq-6.3.0.redhat-187/data/amq/selectorcache.data (No such file or directory)
    at java.io.FileOutputStream.open0(Native Method)[:1.8.0_71]
    at java.io.FileOutputStream.open(FileOutputStream.java:270)[:1.8.0_71]
    at java.io.FileOutputStream.<init>(FileOutputStream.java:213)[:1.8.0_71]
    at java.io.FileOutputStream.<init>(FileOutputStream.java:162)[:1.8.0_71]
    at org.apache.activemq.plugin.SubQueueSelectorCacheBroker.persistCache(SubQueueSelectorCacheBroker.java:217)[162:org.apache.activemq.activemq-osgi:5.11.0.redhat-630187]
    at org.apache.activemq.plugin.SubQueueSelectorCacheBroker.run(SubQueueSelectorCacheBroker.java:256)[162:org.apache.activemq.activemq-osgi:5.11.0.redhat-630187]
    at java.lang.Thread.run(Thread.java:745)[:1.8.0_71]