Prevent a JS error in MergeRequestTabs

When window.location.hash is pointing to a note, e.g. #note_1234, scrollToElement would throw an error because a selector such as .commits #note_1234 doesn't exist, so offset() returned undefined.

This error would prevent subsequent calls from running, which caused the loading spinner to never be hidden.

Now we ensure the selector returns a valid element before trying to scroll to it.

This is the proper fix for !1553 (merged).