keys/types.go#readInfo should check for empty bytes

[ebuchman]

Using this in the cosmos-sdk CLI gives indecipherable error when there's a simple typo in the key name

cosmos-sdk$ basecli keys show mykeyl
ERROR: EOF reading prefix bytes.

https://github.com/tendermint/go-crypto/blob/develop/keys/types.go#L53 should return a better error if the bytes are empty.

[ValarDragon]

This has been fixed

$ gaiacli keys show nonExistantKey
ERROR: Key nonExistantKey not found