Remove assertion that triggers if next == NULL.
Remove assertion that triggers if next == NULL.

schedule() can return null in case of IRQs.

BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,mslovy/barrelfish,kishoredbn/barrelfish,8l/barrelfish,BarrelfishOS/barrelfish,utsav2601/cmpe295A,mslovy/barrelfish,mslovy/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,8l/barrelfish,kishoredbn/barrelfish,mslovy/barrelfish,8l/barrelfish,BarrelfishOS/barrelfish,utsav2601/cmpe295A,8l/barrelfish,kishoredbn/barrelfish,mslovy/barrelfish,utsav2601/cmpe295A,kishoredbn/barrelfish,kishoredbn/barrelfish,utsav2601/cmpe295A,BarrelfishOS/barrelfish,mslovy/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,utsav2601/cmpe295A,8l/barrelfish,utsav2601/cmpe295A,8l/barrelfish,kishoredbn/barrelfish