fix(native): Fix an NPE issue in HttpNativeExecutionTaskResultFetcher (#26550)

## Description
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Fixes a potential NPE issue in HttpNativeExecutionTaskResultFetcher. 
See #22338

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Author: xin-zhang2

presto-spark-base/src/main/java/com/facebook/presto/spark/execution/nativeprocess/HttpNativeExecutionTaskResultFetcher.java

@@ -67,6 +67,8 @@ public class HttpNativeExecutionTaskResultFetcher
 
     private ScheduledFuture<?> scheduledFuture;
 
+    private volatile boolean completed;
+
     private long token;
 
     public HttpNativeExecutionTaskResultFetcher(
@@ -134,6 +136,11 @@ private void throwIfFailed()
 
     private void doGetResults()
     {
+        if (completed && scheduledFuture != null) {
+            scheduledFuture.cancel(false);
+            return;
+        }
+
         if (bufferMemoryBytes.longValue() >= MAX_BUFFER_SIZE.toBytes()) {
             return;
         }
@@ -172,8 +179,11 @@ private void onSuccess(PageBufferClient.PagesResponse pagesResponse)
         }
         token = nextToken;
         if (pagesResponse.isClientComplete()) {
+            completed = true;
             workerClient.abortResultsAsync();
-            scheduledFuture.cancel(false);
+            if (scheduledFuture != null) {
+                scheduledFuture.cancel(false);
+            }
         }
         if (!pages.isEmpty()) {
             synchronized (taskHasResult) {