The value of delegator shares on a validator couldn't be zero once in a while

[banishee]

Summary of Bug

1. Describe

A private network was running on my own test machines, then I found a situation that looks wired. The value of delegator shares on a validator couldn't be zero once in a while, even if there are none of delegators adding shares on that validator

For example:

• Step 1: query the value of delegator shares on one of validators

$ okchaincli query staking validators 
[
  {
    "operator_address": "okchainvaloper1qy4e490rwctehf2xvvjqsmhn4cp4fmyepadax6",
 ……
    "jailed": true,
    "status": 0,
    "delegator_shares": "217679375.96579041",   
 ……
    "unbonding_height": "83804",
    "unbonding_time": "2020-07-20T09:04:40.574657261Z",
    "min_self_delegation": "10000.00000000"
  },
 ……
]

• Step 2: query the list of delegators who added shares to the validator, but the result may be null

$ okchaincli query staking shares-added-to okchainvaloper1qy4e490rwctehf2xvvjqsmhn4cp4fmyepadax6
null

2. Impact

• Some validators who are destroyed can't be removed from the validator set
• the value of delegator share may increase for no reason

3. Recover

Version

before https://github.com/okex/okchain/commit/5af033480fbfa89ebdd34578e68bf08cb2cedaf4, I suspect

Steps to Reproduce

• Actually, there is only a very tiny possibility of reproducing this situation
• I am not sure it was my own problem with the local config environment of okchaincli, or not.

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