revise get_next_core to return always a valid core_id
revise get_next_core to return always a valid core_id

RWTH-OS/HermitCore,stlankes/HermitCore,stlankes/HermitCore,stlankes/HermitCore,RWTH-OS/HermitCore,RWTH-OS/HermitCore,stlankes/HermitCore,stlankes/HermitCore,stlankes/HermitCore,RWTH-OS/HermitCore,RWTH-OS/HermitCore,RWTH-OS/HermitCore