When I was young and immortal (and poor), I often rode parts with known cracks, and I definitely would have ridden that stem. I'm with
scarlson and
rhm , the crack is unlikely to fail catastrophically. But I've turned into a scaredy-cat since I got old and breakable. Knowing it's there would bum me out on high-speed downhills.
Here's a TTT that failed on me when the bike fell over:
Totally different location than OP's crack though.
Mark B