[TW-1810] Completing a recurring task sometimes creates an empty entry in data and wipes some other entries

_Roy Zuo on 2016-06-01T23:52:42Z says:_

It does not happen every time, but when it does, the result is messy.

It creates an empty entry "[]" in `completed.data`, and it deletes *a lot* of entries from `pending.data`. Please check the attachment for command outputs of the symptom.

I am not sure if this is directly associated my marking a recurring task as completed, but this is the second time I see it after upgrading to 2.5.1.