Cap maximum renter payment in RenewAndClear

Currently, the host requires the renter to pay BaseRPCPrice to renew-and-clear a contract. This means that if the contract contains less than BaseRPCPrice, the renter is unable to renew it. Presumably, it's in the host's interest to just take whatever's left, since the real value is in the renewed contract anyway.

This has been observed at least once in the wild: https://github.com/lukechampine/us/issues/75

I imagine it has happened to siad renters as well, since siad will automatically attempt to renew a contract when it runs out of funds.