Replaced FBReportMustFix with IGFailAssert (#1653)

Summary:
## Changes in this pull request

`FBReportMustFix` isn't recognized by the public version of IGListKit. It is wrapped by `IGFailAssert` to abstract error tracking between the public and private versions, and so this change was causing the GitHub repo to stop compiling.

This diff makes the appropriate change and confirms the unit tests are passing again.

### Checklist

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/main/.github/CONTRIBUTING.md)

Pull Request resolved: https://github.com/instagram/IGListKit/pull/1653

Test Plan:
Compiling again!

 {F1984942550}

Differential Revision: D91551520

Pulled By: TimOliver

fbshipit-source-id: bdc9f786092853252011c551ce6d214a01445c63

Source/IGListKit/IGListCollectionViewLayout.mm

@@ -301,7 +301,7 @@ static void adjustZIndexForAttributes(UICollectionViewLayoutAttributes *attribut
     CGRect frame = _sectionData[indexPath.section].itemBounds[indexPath.item];
     // Avoid setting frames with nan values
     if (isnan(frame.origin.x) || isnan(frame.origin.y) || isnan(frame.size.width) || isnan(frame.size.height)) {
-        FBReportMustFix(@"IGListCollectionViewLayout encountered nan frame values for indexPath %@. Original frame: %@", indexPath, NSStringFromCGRect(frame));
+        IGFailAssert(@"IGListCollectionViewLayout encountered nan frame values for indexPath %@. Original frame: %@", indexPath, NSStringFromCGRect(frame));
         return nil;
     }
     attributes.frame = frame;