zstd: free Decoder resources when Reset is called with a nil io.Reader

[mostynb]

Attempt at fixing #296.

[mostynb]

(I will dig into the test failures in the next few days.)

[klauspost]

Thanks for the contribution.

Let's update the docs as well.

// Reset will reset the decoder the supplied stream after the current has finished processing.
// Note that this functionality cannot be used after Close has been called.
// Reset can be called with a nil reader to release references to the previous reader.
// When sending a nil reader no other operations than Reset or DecodeAll or Close should be used.
func (d *Decoder) Reset(r io.Reader) error {

A test that also tests the basic functionality, ie create a Reader, Reset with nil and check that reads returns the expected error and finally that it can be used with a proper reader would be great.

[klauspost]

Thanks. Looks good. I will give it a final look through and merge when I have the time.