staking: Fix potential division-by-zero

`v.DelegatorShares` can be 0 so don't allow division in that case.

x/staking/types/validator.go

@@ -316,6 +316,9 @@ func (v Validator) InvalidExRate() bool {
 
 // calculate the token worth of provided shares
 func (v Validator) TokensFromShares(shares sdk.Dec) math.LegacyDec {
+	if v.DelegatorShares.IsZero() {
+		return math.LegacyZeroDec()
+	}
 	return (shares.MulInt(v.Tokens)).Quo(v.DelegatorShares)
 }