Implement minetest.ipc_cas()

src/script/common/c_packer.cpp

@@ -507,6 +507,7 @@ PackedValue *script_pack(lua_State *L, int idx)
 
 void script_unpack(lua_State *L, PackedValue *pv)
 {
+	assert(pv);
 	// table that tracks objects for keep_ref / PUSHREF (key = instr index)
 	lua_newtable(L);
 	const int top = lua_gettop(L);