[SPARK-18126][SPARK-CORE] getIteratorZipWithIndex accepts negative value as index

## What changes were proposed in this pull request?

(Please fill in changes proposed in this fix)

`Utils.getIteratorZipWithIndex` was added to deal with number of records > 2147483647 in one partition.

method `getIteratorZipWithIndex` accepts `startIndex` < 0, which leads to negative index.

This PR just adds a defensive check on `startIndex` to make sure it is >= 0.

## How was this patch tested?

Add a new unit test.

Author: Miao Wang <miaowang@Miaos-MacBook-Pro.local>

Closes #15639 from wangmiao1981/zip.

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

@@ -1765,6 +1765,7 @@ private[spark] object Utils extends Logging {
    */
   def getIteratorZipWithIndex[T](iterator: Iterator[T], startIndex: Long): Iterator[(T, Long)] = {
     new Iterator[(T, Long)] {
+      require(startIndex >= 0, "startIndex should be >= 0.")
       var index: Long = startIndex - 1L
       def hasNext: Boolean = iterator.hasNext
       def next(): (T, Long) = {

core/src/test/scala/org/apache/spark/util/UtilsSuite.scala

@@ -401,6 +401,9 @@ class UtilsSuite extends SparkFunSuite with ResetSystemProperties with Logging {
     assert(iterator.toArray === Array(
       (0, -1L + Int.MaxValue), (1, 0L + Int.MaxValue), (2, 1L + Int.MaxValue)
     ))
+    intercept[IllegalArgumentException] {
+      Utils.getIteratorZipWithIndex(Iterator(0, 1, 2), -1L)
+    }
   }
 
   test("doesDirectoryContainFilesNewerThan") {