[BUG] Potenial memory leak when encountering disk unhealthy (#370)

### What changes were proposed in this pull request?

Fix potential memory leak when encountering disk unhealthy

### Why are the changes needed?

When encountering disk unhealthy and exceed the timeout of pendingDataShuffleFlushEvent, it will release memory. But in current codebase, it wont release the data reference and cause the memory leak.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?
No need.

server/src/main/java/org/apache/uniffle/server/ShuffleFlushManager.java

@@ -208,12 +208,8 @@ private void flushToFile(ShuffleDataFlushEvent event) {
       // just log the error, don't throw the exception and stop the flush thread
       LOG.error("Exception happened when process flush shuffle data for " + event, e);
     } finally {
-      ShuffleBuffer shuffleBuffer = event.getShuffleBuffer();
-      if (shuffleBuffer != null) {
-        shuffleBuffer.clearInFlushBuffer(event.getEventId());
-      }
+      cleanupFlushEventData(event);
       if (shuffleServer != null) {
-        shuffleServer.getShuffleBufferManager().releaseMemory(event.getSize(), true, false);
         long duration = System.currentTimeMillis() - start;
         if (writeSuccess) {
           LOG.debug("Flush to file success in " + duration + " ms and release " + event.getSize() + " bytes");
@@ -310,14 +306,22 @@ void processPendingEvents() throws Exception {
     addPendingEventsInternal(event);
   }
 
-  private void dropPendingEvent(PendingShuffleFlushEvent event) {
-    ShuffleServerMetrics.counterTotalDroppedEventNum.inc();
+  private void cleanupFlushEventData(ShuffleDataFlushEvent event) {
+    ShuffleBuffer shuffleBuffer = event.getShuffleBuffer();
+    if (shuffleBuffer != null) {
+      shuffleBuffer.clearInFlushBuffer(event.getEventId());
+    }
     if (shuffleServer != null) {
       shuffleServer.getShuffleBufferManager().releaseMemory(
-          event.getEvent().getSize(), true, false);
+          event.getSize(), true, false);
     }
   }
 
+  private void dropPendingEvent(PendingShuffleFlushEvent event) {
+    ShuffleServerMetrics.counterTotalDroppedEventNum.inc();
+    cleanupFlushEventData(event.getEvent());
+  }
+
   @VisibleForTesting
   void addPendingEvents(ShuffleDataFlushEvent event) {
     addPendingEventsInternal(new PendingShuffleFlushEvent(event));