fix out of range error

[fozzysec]

Traceback (most recent call last):
  File "btmspanner.py", line 13, in <module>
    sys.exit(merge_utxo.main())
  File "D:\Users\fozzy\Downloads\bytom-spanner\utxomerger\merge_utxo.py", line 121, in main
    utxo_mergelist.append(utxolist[i])