[storage] fix that StateKey::decode() can panic
[storage] fix that StateKey::decode() can panic

Closes: #965

aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core