Merge pull request #718 from ethereum/empty-base-reward

handle get_base_reward when no previous balance

specs/core/0_beacon-chain.md

@@ -2169,6 +2169,9 @@ First, we define some additional helpers:
 
 ```python
 def get_base_reward(state: BeaconState, index: ValidatorIndex) -> Gwei:
+    if get_previous_total_balance(state) == 0:
+        return 0
+
     adjusted_quotient = integer_squareroot(get_previous_total_balance(state)) // BASE_REWARD_QUOTIENT
     return get_effective_balance(state, index) // adjusted_quotient // 5
 ```