Read on OverlayStream without a known size fails

[yunzheng]

When OverlayStream is initialized without specifying the size, the read() call will return an empty buffer.

The following test case fails:

def test_overlay_stream_no_size() -> None:
    data = b"1234567890"
    buf = io.BytesIO(data)

1. Initialize without a size
       overlay = stream.OverlayStream(buf)

1. read exactly the size of the buffer
       overlay.seek(0)
       assert overlay.read(10) == data

1. read until EOF
       overlay.seek(0)
       assert overlay.read() == data

1. read until EOF using -1
       overlay.seek(0)
       assert overlay.read(-1) == data