Don't try bonding zero liquidity in RewardReinvestor

[code423n4]

Handle

pmerkleplant

Vulnerability details

Impact

Function RewardReinvestor::_bondAccount tries to bond liquidity to an account,
even though it is known whether the liquidity is zero.

Proof of Concept

The return value liquidityCreated in line 105 can be zero.
The following function call, bondToAccount(), then reverts with "Cannot bond 0".

Recommended Mitigation Steps

Gas could be saved if the function would revert earlier, i.e. in line 106,
if the liquidityCreated is zero.

[GalloDaSballo]

In principle I agree with the finding, failing early would save the cost of the approve and the CALL
In practice the refactoring may be more complex than necessary, so up to the sponsor to actually implement the improvement