[SPARK-27018][CORE] Fix incorrect removal of checkpointed file in PeriodicCheckpointer

## What changes were proposed in this pull request?
remove the oldest checkpointed file only if next checkpoint exists.
I think this patch needs back-porting.

## How was this patch tested?
existing test

local check in spark-shell with following suite:
```
import org.apache.spark.ml.linalg.Vectors
import org.apache.spark.ml.classification.GBTClassifier

case class Row(features: org.apache.spark.ml.linalg.Vector, label: Int)

sc.setCheckpointDir("/checkpoints")
val trainingData = sc.parallelize(1 to 2426874, 256).map(x => Row(Vectors.dense(x, x + 1, x * 2 % 10), if (x % 5 == 0) 1 else 0)).toDF
val classifier = new GBTClassifier()
  .setLabelCol("label")
  .setFeaturesCol("features")
  .setProbabilityCol("probability")
  .setMaxIter(100)
  .setMaxDepth(10)
  .setCheckpointInterval(2)

classifier.fit(trainingData)
```

Closes apache#24870 from zhengruifeng/ck_update.

Authored-by: zhengruifeng <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: zhengruifeng

core/src/main/scala/org/apache/spark/util/PeriodicCheckpointer.scala

@@ -100,7 +100,7 @@ private[spark] abstract class PeriodicCheckpointer[T](
       var canDelete = true
       while (checkpointQueue.size > 1 && canDelete) {
         // Delete the oldest checkpoint only if the next checkpoint exists.
-        if (isCheckpointed(checkpointQueue.head)) {
+        if (isCheckpointed(checkpointQueue(1))) {
           removeCheckpointFile()
         } else {
           canDelete = false