I would not want to rely on a single bolt clamping the steerer. I see three methods to fix it:

1. Buy a longer bolt of the same diameter and a matching nut. Thread the bolt all the way through and add the nut to the exposed end. An "acorn" nut would be best to protect your legs from any exposed threads.

2. Drill and tap the damaged hole to the next size up. Assuming the stock bolt is an M5x.8 you would have to drill and tap it to M6x1.0.

3. Buy a new stem and use a torque wrench when you install it so the same thing doesn't happen again.