Disallow minting with negative amounts in the mint-lock example.
Why
There isn't really a good reason to generically support minting with negative amounts. Due to the tracking of the limit inside the mint-lock contract it isn't well defined or obvious what the limits behavior should be with negative mints. For example, should it reduce the limit consumed, or keep it as it is. For the most part token mint functions usually don't support minting negative amounts.
What
Disallow minting with negative amounts in the mint-lock example.
Why
There isn't really a good reason to generically support minting with negative amounts. Due to the tracking of the limit inside the mint-lock contract it isn't well defined or obvious what the limits behavior should be with negative mints. For example, should it reduce the limit consumed, or keep it as it is. For the most part token mint functions usually don't support minting negative amounts.
Close #295