Closing a runtime already in CLOSING state should be a NOP.

cdstar-runtime/src/main/java/de/gwdg/cdstar/runtime/CDStarRuntime.java

@@ -302,7 +302,7 @@ public class CDStarRuntime implements RuntimeContext, Closeable {
 
 	@Override
 	public void close() {
-		if (state.is(State.CLOSED))
+		if (state.is(State.CLOSING, State.CLOSED))
 			return;
 
 		if (state.is(State.NEW, State.INITIALIZED)) {