Fix setValueCount in splitAndTransfer of Variable Width Vector.

We need to use the split length as the value count of the target
vector. We are incorrectly using the value count of the current
vector for the target vector. Thus the latter ends up asking
for a realloc when it didn't really need extra memory.

Author: siddharthteotia

java/vector/src/main/java/org/apache/arrow/vector/BaseVariableWidthVector.java

@@ -711,9 +711,7 @@ public void splitAndTransferTo(int startIndex, int length,
     splitAndTransferValidityBuffer(startIndex, length, target);
     splitAndTransferOffsetBuffer(startIndex, length, target);
     target.setLastSet(length - 1);
-    if (this.valueCount > 0) {
-      target.setValueCount(this.valueCount);
-    }
+    target.setValueCount(length);
   }
 
   /*