Fix math domain error

[yaakiyu]

when memory bytes was't gotten collectly

Rationale

When memory bytes was't gotten collectly, the jsk command wouldn't run.

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/dist-packages/discord/ext/commands/core.py", line 168, in wrapped
    ret = await coro(*args, **kwargs)
  File "/usr/local/lib/python3.9/dist-packages/jishaku/features/root_command.py", line 102, in jsk
    f"{natural_size(mem.uss)} of which unique to this process.")
  File "/usr/local/lib/python3.9/dist-packages/jishaku/features/root_command.py", line 46, in natural_size
    power = int(math.log(size_in_bytes, 1024))
ValueError: math domain error

I checked mem.uss and returned 0.
Maybe because the bot is runnning on WSL, but I want to run jsk command.
So I created a pull request.
I'm Japanese, so my English may look clumsy. Sorry.

Summary of changes made

Only put try and except ValueError: power=0.

Checklist

• This PR changes the jishaku module/cog codebase
  • These changes add new functionality to the module/cog
  • These changes fix an issue or bug in the module/cog
  • I have tested that these changes work on a production bot codebase
  • I have tested these changes against the CI/CD test suite
  • I have updated the documentation to reflect these changes
• This PR changes the CI/CD test suite
  • I have tested my suite changes are well-formed (all tests can be discovered)
  • These changes adjust existing test cases
  • These changes add new test cases
• This PR changes prose (such as the documentation, README or other Markdown/RST documents)
  • I have proofread my changes for grammar and spelling issues
  • I have tested that any changes regarding Markdown/RST syntax result in a well formed document