CR: Enforce "one time only" with a bool, not a cell ref

Since cell's can be re-used, at a later point, we might not be
referencing the data we intend. Since all we want to do is enforce a
"one time only" check, just use a bool.

// FREEBIE

Signal/src/ViewControllers/GifPicker/GifPickerViewController.swift

@@ -40,7 +40,7 @@ class GifPickerViewController: OWSViewController, UISearchBarDelegate, UICollect
     var noResultsView: UILabel?
     var searchErrorView: UILabel?
     var activityIndicator: UIActivityIndicatorView?
-    var selectedCell: UICollectionViewCell?
+    var hasSelectedCell: Bool = false
     var imageInfos = [GiphyImageInfo]()
 
     var reachability: Reachability?
@@ -312,11 +312,11 @@ class GifPickerViewController: OWSViewController, UISearchBarDelegate, UICollect
             return
         }
 
-        guard self.selectedCell == nil else {
+        guard self.hasSelectedCell == false else {
             owsFail("\(TAG) Already selected cell")
             return
         }
-        self.selectedCell = cell
+        self.hasSelectedCell = true
 
         // Fade out all cells except the selected one.
         let maskingView = OWSBezierPathView()