Prevents NSObjectInaccessibleException due to nuked Comment

Fixes #2690

/cc @koke may i bother you with a quick review? (thank you!)

Psst - this is a warning. 
```
/Users/sendhil/Projects/WordPress/WordPress-iOS/WordPress/Classes/Services/CommentService.m:518:38: Incompatible pointer types initializing 'Comment *' with an expression of type 'NSManagedObject *'
```

cc @jleandroperez , @koke