[bug] Resetting of value to non-zero when burning last token

[mg6maciej]

When burn is called on the last token, code in removeNFToken will not clear idToOwnerIndex for it.
Order of these statements should be changed, but even better to use check for last token mentioned in previous issue #104.

    idToOwnerIndex[_tokenId] = 0;
    idToOwnerIndex[lastToken] = tokenToRemoveIndex;

[MoMannn]

I'm kind of not sure which approach would be better here.

I would change the order because it is unlikely that an event of burning all the tokens happens often if at all so the check mentioned it #104 would consume more gas overall.

But on the other hand if there would be contract that constantly burns all the tokens the gas price will be much higher for it.

@mg6maciej @xpepermint @lknix Thoughts?

[lknix]

I'd go for simplicity (changing the order) unless gas difference justifies it.

[lknix]

@mg6maciej current fix: #110