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:`.

test/functional/api/command_spec.lua

@@ -274,6 +274,25 @@ describe('nvim_create_user_command', function()
     eq(42, api.nvim_eval('g:command_fired'))
   end)
 
+  it('does not leak `preview` LuaRef on invalid `cmd`', function()
+    local released = exec_lua(function()
+      local weak = setmetatable({}, { __mode = 'v' })
+      for i = 1, 10 do
+        local cb = function() end
+        weak[i] = cb
+        pcall(vim.api.nvim_create_user_command, 'Bogus' .. i, {}, { preview = cb })
+      end
+      collectgarbage('collect')
+      collectgarbage('collect')
+      local n = 0
+      for _ in pairs(weak) do
+        n = n + 1
+      end
+      return n
+    end)
+    eq(0, released)
+  end)
+
   it('works with Lua functions', function()
     exec_lua [[
       result = {}