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: 
In the case that User.find_by_tracking_code(like.tracking_code) is nil, ```like.user = User.find_by_tracking_code(like.tracking_code)``` will not raise an exception because it will think you want to set the user association to nil.  So the Like will stay a zombie record. :cry: