IndexError in BasketSummaryView - line.has_discount vs. basket.applied_offers

We hit this "list index out of range" error recently in BasketSummaryView._process_basket_lines, but the offending line of code has not been touched in 2 years. The logic assumes that if a basket line's has_discount property is true then it's applied_offers() will have at least on item. The OpsGenie alert for this was only a a P1 because the 12 occurrences of this constituted a significant percentage of all requests over a 5 minute period.

the offending line:
https://github.com/edx/ecommerce/blame/ee7e9d1c1daa746f53bfb60a6babac4c3e59cdec/ecommerce/extensions/basket/views.py#L286

the OpsGenie alert:
https://app.opsgenie.com/alert#/show/c3c8e35b-a775-4e9b-aaf5-f09333d93dc2-1537726615656/details

the NewRelic link
https://rpm.newrelic.com/accounts/88178/applications/5519285/filterable_errors#/table?top_facet=transactionUiName&barchart=barchart