[Downloader] Cog update fails if cogs are already up-to-date

[jack1142]

Command bugs

Command name

[p]cog update* commands

What cog is this command from?

Downloader

What is the issue?

If cogs are already up-to-date, Downloader errors:

Exception in command 'cog updatetoversion'
Traceback (most recent call last):
  File "venv\lib\site-packages\discord\ext\commands\core.py", line 79, in wrapped
    ret = await coro(*args, **kwargs)
  File "venv\lib\site-packages\redbot\cogs\downloader\downloader.py", line 802, in _cog_updatetoversion
    await self._cog_update_logic(ctx, repo=repo, rev=rev, cogs=cogs)
  File "venv\lib\site-packages\redbot\cogs\downloader\downloader.py", line 909, in _cog_update_logic
    for module in cogs_to_update + libs_to_update
UnboundLocalError: local variable 'cogs_to_update' referenced before assignment

---

Regression introduced by #3106