Merge branch '9829-fix-roadmap-sort-typo' into 'master'

Fix typo in Roadmap sorting causing scrolling to fail on null dates

Closes #9829

See merge request gitlab-org/gitlab-ee!9604

ee/app/assets/javascripts/roadmap/utils/roadmap_utils.js

@@ -428,7 +428,7 @@ export const sortEpics = (epics, sortedBy) => {
       }
 
       if (b.startDateUndefined) {
-        aDate = pastDate;
+        bDate = pastDate;
       } else {
         bDate = b.startDateOutOfRange ? b.originalStartDate : b.startDate;
       }