verify the ignored fee is still 0

[antiochp]

Resolves #2859.

While we continue to support kernel features using protocol version 1 -

• read but ignore fee on coinbase kernels
• but verify these ignored bytes are indeed 0

[antiochp]

@garyyu @tromp This is all that should be required here right?
When we read the bytes we still discard the fee and lock_height if not applicable, but we need to verify the bytes are all 0 before we discard them.

[tromp]

I prefer a test

if lock_height/fee != 0

instead of > 0, because the latter has me wondering about whether it could be negative (they can't because they're unsigned), whereas comparing for equality is immediately clear.

[tromp]

otherwise, looks fine.