RentableNFT finishRenting vulnerability to return transferred token

[SamWarden]

RentableNFT finishRenting allows a lord to return a transferred token from the new owner if he is the last renter. A possible RentableNFT finishRenting abuse algorithm:

1. An owner of a token rentOut it to a renter.
2. Waits for the rental to expire and returns the token back.
3. The owner gives/sells the token again to the same renter. (Transfer)
4. Despite the fact that now the real owner of the token is the renter, the former owner has an opportunity to return the token back at any time through the same finishRenting.

To fix this, just add a check that the token is rented before returning it to its lord.

[SamWarden]

A test to demonstrate abuse of the vulnerability https://github.com/SamWarden/sol-like-a-pro/blob/11e4669cf6e4c3503a506739164e66b81ebc7ba6/test/rentable-nft/RentableNFT.test.ts#L126

[SamWarden]

This is my fix option #3