Fix reference leak in ReifyTimestampAndWindowsParDoFnFactory

Author: Abacn

runners/google-cloud-dataflow-java/worker/src/main/java/org/apache/beam/runners/dataflow/worker/ReifyTimestampAndWindowsParDoFnFactory.java

@@ -19,6 +19,7 @@
 
 import static org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.Preconditions.checkState;
 
+import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.time.Instants;
 import com.google.api.services.dataflow.model.SideInputInfo;
 import java.util.List;
 import java.util.Map;
@@ -87,9 +88,13 @@ public void processElement(Object untypedElem) throws Exception {
     public void processTimers() {}
 
     @Override
-    public void finishBundle() throws Exception {}
+    public void finishBundle() throws Exception {
+      this.receiver = null;
+    }
 
     @Override
-    public void abort() throws Exception {}
+    public void abort() throws Exception {
+      this.receiver = null;
+    }
   }
 }