Gauge admin can add faulty reward tokens

[nventuro]

On LiquidityGaugeV5, the add_reward function does not check that _distributor is not the zero address. This is important, because the gauge assumes reward tokens with a zero distributor are uninitialized (i.e. not added to the gauge).

A zero distributor means that deposit_reward_token will not be callable, nor can this situation be fixed using set_reward_distributor. Additionally, the admin would then be able to add the same reward token again to the gauge, since add_reward only checks for the current distributor to be unset.

This is not a security concern due to the following:

• if the reward token is only added once, no rewards will be able to be issued for it, and the gauge will behave as if the distributor never sent any tokens
• if the reward token is added multiple times, the internal accounting will not be broken since all reward tokens are fully processed before moving on to the next one (see e.g. _checkpoint_rewards)

This does result in extra gas being used by all users, and one (or more) of the 8 reward token slots being forever burned.

The proposed fix is to simply check that _distributor is not the zero address in add_reward. From what I've seen, this issue is not present just on the V5 gauge but also all past gauges. Care should be taken to not add a reward token with a zero distributor in these.