IF-STRIDE-STAKEIBC-UPDATEDELEGATIONBALANCES fix (#447)

Underflow possible when decreasing staked balance for host zone

# Involved artifacts

- [x/stakeibc/keeper/icacallbacks_undelegate.go](https://github.com/Stride-Labs/stride/blob/v2.0.3/x/stakeibc/keeper/icacallbacks_undelegate.go#L128)

# Description

When undelegating from each validator, host zone's staked balance is being decreased according to undelegation amount of particular validator:

```go
for _, undelegation := range undelegateCallback.SplitDelegations {
		undelegateAmt, err := cast.ToInt64E(undelegation.Amount)
		k.Logger(ctx).Info(fmt.Sprintf("UndelegateCallback, Undelegation: %d, validator: %s", undelegateAmt, undelegation.Validator))
		if err != nil {
			errMsg := fmt.Sprintf("Could not convert undelegate amount to int64 in undelegation callback | %s", err.Error())
			k.Logger(ctx).Error(errMsg)
			return sdkerrors.Wrapf(types.ErrIntCast, errMsg)
		}
		undelegateVal := undelegation.Validator
		success := k.AddDelegationToValidator(ctx, zone, undelegateVal, -undelegateAmt)
		if !success {
			return sdkerrors.Wrapf(types.ErrValidatorDelegationChg, "Failed to remove delegation to validator")
		}
		zone.StakedBal -= undelegation.Amount
	}
```

However, `zone.StakedBal` is defined as `uint64`; therefore, it might underflow if `undelegation.Amount` > `zone.StakedBal`. Undelegation amount for each undelegation is calculated in [GetTargetValAmtsForHostZone](https://github.com/Stride-Labs/stride/blob/v2.0.3/x/stakeibc/keeper/unbonding_records.go#L74) and have been additionaly modified during [overflow](https://github.com/Stride-Labs/stride/blob/v2.0.3/x/stakeibc/keeper/unbonding_records.go#L103-L139) handling.


# Problem Scenarios

If underflow happens, staked balance of particular zone will unexpectedly and quietly become tremendously large having adverse consequences during both, `UndelegateCallback` and `DelegateCallback`.


# Recommendation

Add validation check before decreasing the staked balance amount: 

```go
  if undelegation.Amount > zone.StakedBal {
  // handle incoming underflow
  } else {
    zone.StakedBal -= undelegation.Amount
  }
```

x/stakeibc/keeper/icacallbacks_undelegate.go

@@ -120,7 +120,13 @@ func (k Keeper) UpdateDelegationBalances(ctx sdk.Context, zone types.HostZone, u
 		if !success {
 			return sdkerrors.Wrapf(types.ErrValidatorDelegationChg, "Failed to remove delegation to validator")
 		}
-		zone.StakedBal -= undelegation.Amount
+		if undelegation.Amount > zone.StakedBal {
+			// handle incoming underflow
+			// Once we add a killswitch, we should also stop liquid staking on the zone here
+			return sdkerrors.Wrapf(types.ErrUndelegationAmount, "undelegation.Amount > zone.StakedBal, undelegation.Amount: %d, zone.StakedBal %d", undelegation.Amount, zone.StakedBal)
+		} else {
+			zone.StakedBal -= undelegation.Amount
+		}
 	}
 	k.SetHostZone(ctx, zone)
 	return nil

x/stakeibc/types/errors.go

@@ -47,4 +47,5 @@ var (
 	ErrHostZoneICAAccountNotFound        = sdkerrors.Register(ModuleName, 1537, "host zone's ICA account not found")
 	ErrNoValidatorAmts                   = sdkerrors.Register(ModuleName, 1538, "could not fetch validator amts")
 	ErrMaxNumValidators                  = sdkerrors.Register(ModuleName, 1539, "max number of validators reached")
+	ErrUndelegationAmount                = sdkerrors.Register(ModuleName, 1540, "Undelegation amount is greater than stakedBal")
 )