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:`.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/nvim/api/command.c b/src/nvim/api/command.c
index 50febf1af4..063b99c794 100644
--- a/src/nvim/api/command.c
+++ b/src/nvim/api/command.c
@@ -1279,7 +1279,8 @@ void create_user_command(uint64_t channel_id, String name, Union(String, LuaRef)
     if (uc_add_command(name.data, name.size, rep, argt, def, flags, context, compl_arg,
                        compl_luaref, preview_luaref, addr_type_arg, luaref, force) != OK) {
       api_set_error(err, kErrorTypeException, "Failed to create user command");
-      // Do not goto err, since uc_add_command now owns luaref, compl_luaref, and compl_arg
+      // Do not goto err, since uc_add_command now owns luaref, compl_luaref, preview_luaref,
+      // and compl_arg
     }
   });
 
@@ -1288,6 +1289,7 @@ void create_user_command(uint64_t channel_id, String name, Union(String, LuaRef)
 err:
   NLUA_CLEAR_REF(luaref);
   NLUA_CLEAR_REF(compl_luaref);
+  NLUA_CLEAR_REF(preview_luaref);
   xfree(compl_arg);
 }
 /// Gets a map of global (non-buffer-local) Ex commands.