Fix `add-mint` behaviour in case where `:max-supply` is not specified

[mikera]

Currently (0.8.1) add-mint defaults max supply to 0.

It should allow unlimited minting in this case.

[mikera]

P.S. Workaround is always to set :max-supply explicitly.

This is a good idea anyway, as you almost never want truly unlimited minting. 10^18 is a good value for :max-supply that should be suitable for all reasonable token designs and ensures balances always stay within 64 bits.

2^256-1 is also a sensible number if you want to match the theoretical maximum supply of an ERC20 token.