Add some assertions related to SD buffering.
Add some assertions related to SD buffering.

johnkingsley/aprinter,johnkingsley/aprinter,fixator/aprinter,johnkingsley/aprinter,ambrop72/aprinter,fixator/aprinter,johnkingsley/aprinter,ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,fixator/aprinter,fixator/aprinter,fixator/aprinter,fixator/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,johnkingsley/aprinter