In the actual situation, an Unstoppable Force gets deflected off of an Unmovable Object. Be it over, backwards, or sideways, the force will keep moving without stopping and the object will remain unharmed.
In this case, the Crusader's damage would put the barbarian's HP at -N, where N is any number the player can think of. Unless the Barbarian drowns/suffocates to reset his HP, he will die upon his Frenzy ending.
DnD doesn't have an infinite loop clause that MtG does. If something combos infinitely, the DM is free to resolve the issue at his leisure and proceed with the game as normal.