Fix invalid range in UnsafeSorter.

TODO: write fuzz tests to uncover stuff like this.
Sorting has nice invariants; should be an easy test
to write.

core/src/main/java/org/apache/spark/unsafe/sort/UnsafeSorter.java

@@ -120,7 +120,7 @@ public void insertRecord(long objectAddress) {
   }
 
   public Iterator<KeyPointerAndPrefix> getSortedIterator() {
-    sorter.sort(sortBuffer, 0, sortBufferInsertPosition, sortComparator);
+    sorter.sort(sortBuffer, 0, sortBufferInsertPosition / 2, sortComparator);
     return new Iterator<KeyPointerAndPrefix>() {
       private int position = 0;
       private final KeyPointerAndPrefix keyPointerAndPrefix = new KeyPointerAndPrefix();