Is it safe to par-boil chicken legs, (unthawed), then finish cooking them later?
It is safe, as long as the chicken doesn't stay more than 2 hours in the danger zone (40-140* F / 4-60º C). These 2 hours include preparation, cooking time and storage. 

For further information: Is a partially frozen chicken safe if not immediately cooked at the proper temperature? and Can chicken not completely cooked then cooled be fully cooked later?