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)

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

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

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

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