Check also for the presence of end sentinel

This method causes 500 "undefined method '-' for nil" when user submit track_1 or track_2 value with beginning sentinel, but without the ending one. Submitting it to Authorize.net may result in proper transaction error, instead of 500.
I was thinking about removing sentinels one by one, but it may be a little overkill, especially because nobody noticed that error for at least a couple of years.