Ahahaha! Nice catch! No.. you should not be able to add negative liquidity... the contract will fail here:
https://github.com/hive-engine/hivesmartcontracts/blob/fe0b624bfed9207cb37e82ed51fa9ed3f6ba5884/contracts/marketpools.js#L297

@crypt0gnome FYI, the UI should not allow negative or zero values on add liquidity. Probably the same fix on remove liquidity.

Thanks @masterswatch