Fix java.nio.ByteBuffer.clear() error

Author: tomwhite

src/main/java/org/broadinstitute/hellbender/utils/nio/SeekableByteChannelPrefetcher.java

@@ -1,6 +1,8 @@
 package org.broadinstitute.hellbender.utils.nio;
 
 import com.google.common.base.Stopwatch;
+
+import java.nio.Buffer;
 import java.util.concurrent.ThreadFactory;
 import org.broadinstitute.hellbender.exceptions.GATKException;
 
@@ -142,7 +144,7 @@ public ByteBuffer getBuf() throws ExecutionException, InterruptedException {
 
         public WorkUnit resetForIndex(long blockIndex) {
             this.blockIndex = blockIndex;
-            buf.clear();
+            ((Buffer) buf).clear(); // for Java 11, see https://github.com/jruby/jruby/issues/5450
             futureBuf = null;
             return this;
         }