Bolt issues like this are somewhat common. Hey, things wear out. You can buy that part and replace it at Brownell's, Midway USA and Cheaper Than Dirt if you want. Many places sell those small parts for AR's these days.
But since the gun is modular and other manufacturer's bolts will drop right in and fit in your Bushmaster with no problems. You may want to invest in obtaining a better quallity bolt for your AR. Rock River, Spikes Tactical and Bravo Company (BCM) are a few that come to mind that make good quality AR bolts. I'm sure there are others too.
It may give you peace of mind to know you have a higher quality bolt in your gun, so it won't fail when you need it the most.
I just re-read your original post and realized it's brand new out of the box. In that case I think I would return it to the Manufacturer under warranty for replacement. I'm not sure that's a crack or potential crack though. I still think I'd invest in a better quality bolt anyway. Having a spare might be a good thing. I have a bolt spare parts and repair kit for mine in case something wears out and breaks.