ChecksumObserver sets digest == null on transfer error, leading to NPE's if checksum downloads are retried with same observer instance

See maven-artifact (3.0-snapshot), file DefaultWagonManager.java, line ~506. I had to adjust this to reinit the checksum observer for each download attempt, since transfer error events broadcast to the observer will cause a NPE, making that observer impossible to reuse. The digester instance is set to null instead of reset.