Fix verify_bitfield

[hwwhww]

Issues

1. When committee_size is 16, the bitfield length is 2, and then get_bitfield_bit(bitfield, committee_size) is out of range.
2. If committee_size % 8 != 0, the padding zero should be added from index=committee_size in bitfield.

Fixes

1. Only check the padding zeros when committee_size % 8 != 0, otherwise it will trigger IndexError.
2. Check the zero padding bits from the bitfield index committee_size. (it was starting from committee_size + 1)