This type of bankruptcy is a reorganization (chapter 11) not a liquidation (chapter 7). I wouldn't worry too much at this point. I'm pretty certain that I read here on FT the other day that there has never been a situation where FF miles have become worthless due to bankruptcy.

Even if it comes to a worst-case scenario and DL liquidates, the FF program would be a highly desirable acquisition for another airline as it would bring in customers such as yourself who already have miles invested in that program.