Add check for T_NONE in rb_gc_mark_weak

This commit adds a check for T_NONE in rb_gc_mark_weak, just like
gc_mark_ptr. This will help debugging.

gc.c

@@ -6903,6 +6903,11 @@ rb_gc_mark_weak(VALUE *ptr)
 
     GC_ASSERT(objspace->rgengc.parent_object == 0 || FL_TEST(objspace->rgengc.parent_object, FL_WB_PROTECTED));
 
+    if (UNLIKELY(RB_TYPE_P(obj, T_NONE))) {
+        rp(obj);
+        rb_bug("try to mark T_NONE object");
+    }
+
     /* If we are in a minor GC and the other object is old, then obj should
      * already be marked and cannot be reclaimed in this GC cycle so we don't
      * need to add it to the weak refences list. */