Avoid NPE on shutdown, if server was not started.

Kai Reinhard

22.29.2018 5d63ebab603773a9dfe1fb6e8f683b7d783cdfb2

 borgbutler-server/src/main/java/de/micromata/borgbutler/server/Main.java

@@ -116,6 +116,10 @@
    }

    private void _shutdown() {
        if (server == null) {
            // Do nothing (server wasn't started).
            return;
        }
        synchronized (this) {
            if (shutdownInProgress == true) {
                // Another thread already called this method. There is nothing further to do.