contracts/bm: Add assertions before updating rewards/fees

Should hopefully prevent any other similar accounting problems
from allowing exploiting the minter balance.

Author: victorges

contracts/bonding/BondingManager.sol

@@ -364,6 +364,8 @@ contract BondingManager is ManagerProxyTarget, IBondingManager {
                 delegatorsRewards.add(totalStake)
             );
         }
+        // Make sure there's no accounting error on transcoder rewards state
+        assert(activeCumulativeRewards < totalStake);
 
         uint256 delegatorsFees = MathUtils.percOf(_fees, earningsPool.transcoderFeeShare);
         uint256 transcoderCommissionFees = _fees.sub(delegatorsFees);
@@ -1487,6 +1489,8 @@ contract BondingManager is ManagerProxyTarget, IBondingManager {
 
         t.activeCumulativeRewards = t.cumulativeRewards;
 
+        // Make sure there's no accounting error on transcoder rewards state
+        assert(t.activeCumulativeRewards < earningsPool.totalStake);
         uint256 transcoderCommissionRewards = MathUtils.percOf(_rewards, earningsPool.transcoderRewardCut);
         uint256 delegatorsRewards = _rewards.sub(transcoderCommissionRewards);
         // Calculate the rewards earned by the transcoder's earned rewards