buffered-reader: Fix returning partial reads ending in errors.

  - Make sure that we return the data we already have in our buffer,
    even though we encountered an IO error while filling it.

  - Notably, the packet parser assumes that data once read can be
    requested through the buffered reader protocol again and again.
    Unfortunately, that was not the case, leading to a panic.

  - As the generic reader is used to implement the buffered reader
    protocol on top of io::Read, this problem affects among other
    things the compression container.  Demonstrate this using test.

  - Fixes #1005.