Change handling of nil value markers in template tables.

Reported by Bernhard M. Wiedemann. #1348 #1155
Fixes from Peter Cawley, Christian Clason, Lewis Russell.

src/lj_tab.c

@@ -194,6 +194,7 @@ GCtab * LJ_FASTCALL lj_tab_dup(lua_State *L, const GCtab *kt)
       Node *next = nextnode(kn);
       /* Don't use copyTV here, since it asserts on a copy of a dead key. */
       n->val = kn->val; n->key = kn->key;
+      if (tvistab(&n->val)) setnilV(&n->val); /* Replace nil value marker. */
       setmref(n->next, next == NULL? next : (Node *)((char *)next + d));
     }
   }