fix 500 when reviewer is deleted with integration tests (#6856)

models/fixtures/comment.yml

@@ -52,3 +52,15 @@
   tree_path: "README.md"
   created_unix: 946684812
   invalidated: true
+
+-
+  id: 7
+  type: 21 # code comment
+  poster_id: 100
+  issue_id: 3
+  content: "a review from a deleted user"
+  line: -4
+  review_id: 10
+  tree_path: "README.md"
+  created_unix: 946684812
+  invalidated: true

models/fixtures/review.yml

@@ -70,3 +70,12 @@
   content: "New review 3 rejected"
   updated_unix: 946684810
   created_unix: 946684810
+
+-
+  id: 10
+  type: 3
+  reviewer_id: 100
+  issue_id: 3
+  content: "a deleted user's review"
+  updated_unix: 946684810
+  created_unix: 946684810