[SPARK-20537][CORE] Fixing OffHeapColumnVector reallocation

## What changes were proposed in this pull request?

As #17773 revealed `OnHeapColumnVector` may copy a part of the original storage.

`OffHeapColumnVector` reallocation also copies to the new storage data up to 'elementsAppended'. This variable is only updated when using the `ColumnVector.appendX` API, while `ColumnVector.putX` is more commonly used.
This PR copies the new storage data up to the previously-allocated size in`OffHeapColumnVector`.

## How was this patch tested?

Existing test suites

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #17811 from kiszk/SPARK-20537.

sql/core/src/main/java/org/apache/spark/sql/execution/vectorized/OffHeapColumnVector.java

@@ -436,28 +436,29 @@ public final class OffHeapColumnVector extends ColumnVector {
   // Split out the slow path.
   @Override
   protected void reserveInternal(int newCapacity) {
+    int oldCapacity = (this.data == 0L) ? 0 : capacity;
     if (this.resultArray != null) {
       this.lengthData =
-          Platform.reallocateMemory(lengthData, elementsAppended * 4, newCapacity * 4);
+          Platform.reallocateMemory(lengthData, oldCapacity * 4, newCapacity * 4);
       this.offsetData =
-          Platform.reallocateMemory(offsetData, elementsAppended * 4, newCapacity * 4);
+          Platform.reallocateMemory(offsetData, oldCapacity * 4, newCapacity * 4);
     } else if (type instanceof ByteType || type instanceof BooleanType) {
-      this.data = Platform.reallocateMemory(data, elementsAppended, newCapacity);
+      this.data = Platform.reallocateMemory(data, oldCapacity, newCapacity);
     } else if (type instanceof ShortType) {
-      this.data = Platform.reallocateMemory(data, elementsAppended * 2, newCapacity * 2);
+      this.data = Platform.reallocateMemory(data, oldCapacity * 2, newCapacity * 2);
     } else if (type instanceof IntegerType || type instanceof FloatType ||
         type instanceof DateType || DecimalType.is32BitDecimalType(type)) {
-      this.data = Platform.reallocateMemory(data, elementsAppended * 4, newCapacity * 4);
+      this.data = Platform.reallocateMemory(data, oldCapacity * 4, newCapacity * 4);
     } else if (type instanceof LongType || type instanceof DoubleType ||
         DecimalType.is64BitDecimalType(type) || type instanceof TimestampType) {
-      this.data = Platform.reallocateMemory(data, elementsAppended * 8, newCapacity * 8);
+      this.data = Platform.reallocateMemory(data, oldCapacity * 8, newCapacity * 8);
     } else if (resultStruct != null) {
       // Nothing to store.
     } else {
       throw new RuntimeException("Unhandled " + type);
     }
-    this.nulls = Platform.reallocateMemory(nulls, elementsAppended, newCapacity);
-    Platform.setMemory(nulls + elementsAppended, (byte)0, newCapacity - elementsAppended);
+    this.nulls = Platform.reallocateMemory(nulls, oldCapacity, newCapacity);
+    Platform.setMemory(nulls + oldCapacity, (byte)0, newCapacity - oldCapacity);
     capacity = newCapacity;
   }
 }