Return nil to avoid crash

Summary:
Logiview: https://www.internalfb.com/logview/instagram_ios_crashes/2ece21699d74d3187e8480e2ec0dce14?trace_tab=0&ds=%7B%22start%22%3A%22-1%20day%22%2C%22end%22%3A%22now%22%7D

This crash happens at line 297 in this file, when the `indexPath` is `nil`
early return when `indexPath == nil` to avoid the crash

Differential Revision: D62513388

fbshipit-source-id: 91aef1ab0474563bf639abb1debf9ca1477e64ed

Source/IGListKit/IGListCollectionViewLayout.mm

@@ -279,6 +279,10 @@ - (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSInde
         return attributes;
     }
 
+    if (indexPath == nil) {
+        return nil;
+    }
+
     // avoid OOB errors
     const NSInteger section = indexPath.section;
     const NSInteger item = indexPath.item;
@@ -474,7 +478,7 @@ - (NSString *)_classNameForDelegate:(id<UICollectionViewDelegateFlowLayout>)dele
 - (void)_calculateLayoutIfNeeded {
     if (_minimumInvalidatedSection == NSNotFound) {
         return;
-    }    
+    }
 
     UICollectionView *collectionView = self.collectionView;
     id<UICollectionViewDelegateFlowLayout> delegate = (id<UICollectionViewDelegateFlowLayout>)collectionView.delegate;