Check that delta is unchanged

Author: ViggoC

vector/src/test/java/org/apache/arrow/vector/util/TestVectorAppender.java

@@ -1076,6 +1076,17 @@ public void testAppendRunEndEncodedVector() {
 
         assertVectorsEqual(expected, target);
       }
+
+      // Check that delta is unchanged.
+      final Field expectedDeltaField = new Field("expectedDelta", reeFieldType, children);
+      try (RunEndEncodedVector expectedDelta =
+          new RunEndEncodedVector(expectedDeltaField, allocator, null)) {
+        expectedDelta.allocateNew();
+        ValueVectorDataPopulator.setVector((IntVector) expectedDelta.getValuesVector(), 3, 4, 5, null);
+        ValueVectorDataPopulator.setVector((IntVector) expectedDelta.getRunEndsVector(), 1, 3, 4, 6);
+        expectedDelta.setValueCount(6);
+        assertVectorsEqual(expectedDelta, delta);
+      }
     }
   }