Possible use-after-free while using `lua_tolstring`

[filonenko-mikhail]

Well, as we discussed in Tarantool-ru telegram chat, this approach could lead to use-after-free (https://pgl.yoyo.org/luai/i/lua_tolstring)

Maybe we should use luaL_ref/luaL_unref here.

Also it would be great to add a test for such case.

Originally posted by @olegrok in #22 (comment)

[filonenko-mikhail]

So, such string till exists in table.
No need to additional ref value.