fix(api): leak preview callback LuaRef in nvim_create_user_command #39357

Problem:
Invalid `nvim_create_user_command` calls can leak the
`preview` callback reference after Neovim has taken ownership of it.

1. build with {a,l}san
2. run:
    ```sh
    <path/to/nvim> --headless -u NONE --clean +'lua
    for i = 1, 100 do
      pcall(vim.api.nvim_create_user_command,
        "some very epic stuff" .. i,
        {}, -- NOTE: this is INVALID (not a function or string)
        { preview = function() end })
    end
    vim.cmd("qa!")
    ' +qa
    ```
3. see:
    ```
    100 lua references were leaked!
    ```

Solution:
Clear `preview_luaref` in `err:`.

0 files changed, 0 insertions, 0 deletions