PrefixEndBytes() should return when len(prefix) == 0

[mossid]

In https://github.com/cosmos/cosmos-sdk/blob/develop/store/types/utils.go#L61

        if prefix == nil {
		return nil
	}

        //...
	
	for {
		if end[len(end)-1] != byte(255) {
			end[len(end)-1]++
                        break
                }
                //...
        }

In the for loop, we access on the slice without checking the length. When the provided argument has length 0 but not nil([]byte{}), it throws a runtime error index out of range. We have to replace the conditional expression to len(prefix) == 0 from the current prefix == nil.

[alexanderbez]

Indeed we should always be checking length on these slices in almost all cases.