Fix test failure, the wrapping really found a bug. It's the same like in SlowCompositeReaderWrapper:

ant test -Dtestcase=TestReaderClosed -Dtestmethod=test -Dtests.seed=-7c9f33395d707a4a:9a8a104377eade3:-6839e18852fad654 -Dtests.multiplier=3 -Dtests.nightly=true -Dargs="-Dfile.encoding=ISO8859-1"
Was found by Jenkins!

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1245770 13f79535-47bb-0310-9956-ffa450edef68

lucene/core/src/java/org/apache/lucene/index/ParallelAtomicReader.java

@@ -191,6 +191,11 @@ public final class ParallelAtomicReader extends AtomicReader {
   @Override
   public Fields fields() {
     ensureOpen();
+    // we cache the inner field instances, so we must check
+    // that the delegate readers are really still open:
+    for (final AtomicReader reader : parallelReaders) {
+      reader.ensureOpen();
+    }
     return fields;
   }