fix concurrency issues in bytesboundedlinkedqueue

Fix NPE in drainTo method
java.lang.NullPointerException
        at java.util.LinkedList.unlink(LinkedList.java:209)
        at java.util.LinkedList.remove(LinkedList.java:524)
        at io.druid.client.cache.BytesBoundedLinkedQueue.drainTo(BytesBoundedLinkedQueue.java:223)
        at io.druid.client.cache.BytesBoundedLinkedQueue.drainTo(BytesBoundedLinkedQueue.java:202)