Bugfix: no more infinite loop because of crates' notification

src/killer_sokoban/AbstractField.java

@@ -59,7 +59,7 @@ public abstract class AbstractField {
 		if(!isCorner) return;
 		
 		for(AbstractField neighbor : neighbours.values()) {
-			neighbor.setToCorner();
+			if(!neighbor.isCorner) neighbor.setToCorner();
 		}
 	}