Fix recurseHollow behaving incorrectly

by yet again turning a mutable bv3 into an immutable one

Author: eztaK-red

worldedit-core/src/main/java/com/sk89q/worldedit/EditSession.java

@@ -3664,7 +3664,7 @@ private void recurseHollow(Region region, BlockVector3 origin, Set<BlockVector3>
                 }
 
                 for (BlockVector3 recurseDirection : recurseDirections) {
-                    queue.add(current.add(recurseDirection));
+                    queue.add(current.toImmutable().add(recurseDirection));
                 }
             }
         }