ERC-721: Check ownership in transfer_token_from

[zgrannan]

Summary

• [n] y/n | Does it introduce breaking changes?
• [n] y/n | Is it dependant on the specific version of cargo-contract or pallet-contracts?

The ERC-721 Specification states that transferFrom should throw if from is not the owner of the token. The current implementation doesn't check that. As a result, it's possible for a caller A who is allowed to transfer B's token T, can call transferFrom(C, A, T) without any problem. A side effect is that the balance of C will be decremented instead of B.

The transfer() function also has a similar issue: it seems the intention is to transfer the token from the caller, but the ownership is never checked explicitly. If the caller uses the id of an approved (not owned) token, it will incorrectly decrement the caller's balance.

This PR addresses these issues by adding an ownership check in transfer_token_from. Tests for transfer_from and transfer have been added.

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have added an entry to CHANGELOG.md
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules No dependent changes

[ascjones]

LGTM