@SuppressFBWarnings(value = "EI_EXPOSE_REP", justification = "Expose internal rep for efficiency")
  public byte[] getValPreallocatedBytes() {
    return currentValue;
  }
Ensures that we have space allocated for the next value, which has size
length bytes.

Updates currentValue and currentOffset for this value.

Always use before getValPreallocatedBytes, getValPreallocatedStart.
setValPreallocated must be called to actually reserve the bytes.
storage-api/src/java/org/apache/hadoop/hive/ql/exec/vector/BytesColumnVector.java