destroy zombie likes when likable is invalid - Bounty 456

[BUG: ActiveRecord::RecordInvalid: Validation failed: Likable can't be blank - Bounty 456](https://assembly.com/coderwall/bounties/456)
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The code will now handle when the zombie like has an invalid likable :smile_cat: 
shouldn't be `like.user = User.find_by_tracking_code(like.tracking_code)` ?