There are really two ways for those little microbes to make you sick. One is through infection, the other through intoxication. In an infection it is the microbes themselves that make you sick, and yes boiling the stock for a period of time would kill them off. But an intoxication is where the microbes make you sick from the waste products they produce, through feeding, respiration and reproduction. These toxic substances are not always destroyed by boiling. One example is the botulism toxin. It will survive most normal boilings, and there are many others. Chances are your stock would be safe, those little bugs are not that common, but why risk it. The few dollars and little time you spent making it, doesn't justify making yourself or someone else sick. And the potential for really making someone sick, or even killing them is there. So your safest bet is , when in doubt throw it out.