don't try to update rep for the nil user

[cellio]

A moderator tried to delete an answer by a deleted user and got a 500 error, specifically:

    NoMethodError: undefined method `reputation' for nil:NilClass
        user.update(reputation: user.reputation - Vote.total_rep_change(votes))
                                    ^^^^^^^^^^^

/var/apps/qpixel/app/models/post.rb:264:in `modify_author_reputation'
/var/apps/qpixel/app/controllers/posts_controller.rb:402:in `delete'

I added a guard against trying to change the rep for a deleted user. This might not be the best way to do it. Also, I was not able to reproduce the bug in a local dev environment, so it's possible this isn't really the cause of the error. But server errors are bad, so let's try to fix this one.

[cellio]

@Oaphi I applied @ArtOfCode- 's change. The key seems to have been destroyed (rather than deleted) accounts. After I 418ed myself and figured out how to do the db surgery to get back to a working dev environment, I think this works now.

[Oaphi]

LGTM!