[Gridmix] A potential divide by zero in AvgRecordFactory

public AvgRecordFactory(long targetBytes, long targetRecords,
      Configuration conf, int minSpilledBytes) {
    this.targetBytes = targetBytes;
    this.targetRecords = targetRecords <= 0 && this.targetBytes >= 0
      ? Math.max(1,
          this.targetBytes / conf.getInt(GRIDMIX_MISSING_REC_SIZE, 64 * 1024))
      : targetRecords;
    final long tmp = this.targetBytes / this.targetRecords;
    ...
}