Fix heap use after free

In a lua script, if object was created inside a container, it
might've merged with another object.

Also prevent stacking, lighting, and burying contained objects.

src/sp_lev.c

@@ -2239,7 +2239,7 @@ struct mkroom *croom;
 
             remove_object(otmp);
             if (cobj) {
-                (void) add_to_container(cobj, otmp);
+                otmp = add_to_container(cobj, otmp);
                 cobj->owt = weight(cobj);
             } else {
                 obj_extract_self(otmp);
@@ -2324,11 +2324,11 @@ struct mkroom *croom;
         }
     }
 
+    if (!(o->containment & SP_OBJ_CONTENT)) {
         stackobj(otmp);
 
-    if (o->lit) {
+        if (o->lit)
             begin_burn(otmp, FALSE);
-    }
 
         if (o->buried) {
             boolean dealloced;
@@ -2339,6 +2339,7 @@ struct mkroom *croom;
             }
         }
     }
+}
 
 /*
  * Create an altar in a room.