storage.ReadAt doesn't read with empty arrays

[benjlai]

I think the line:
l := int64(len(b))
should be
l := int64(cap(b))
i.e. the length of data copied should be based on the capacity of the array and not how much it has already been filled by other data.

[anz-rfc]

here's the analogous operation in standard library docco:

https://golang.org/pkg/os/#File.ReadAt

ReadAt reads len(b) bytes from the File starting at byte offset off. It returns the number of bytes read and the error, if any. ReadAt always returns a non-nil error when n < len(b). At end of file, that error is io.EOF.

i assume that go-billy needs to implement the same behaviour for consistency with standard library filesystem.

if b is an array (not a slice) then len(b) and cap(b) wil be equal, and if b is an empty array it is not possible for ReadAt to read anything without allocating, which is the responsibility of the caller