Closes#6540
In #6221 there was a mistake in calculating which folds need to be
re-ordered. When there are no folds after those that have been adjusted,
then `move_end` remains 0. This results in reverse_fold_order()
swapping folds that have been adjusted with uninitialised folds in the
remainder of the grow array.
Add a check in foldMoveRange() to account for this case.