In 40 years of shooting, I have never heard that this was a "problem" with any Model 70s.
The truth of the matter is, though, that when you have hundreds of thousands of rifles being mass produced by machinery every year, there is ALWAYS going to be a tiny tiny percentage that have a manufacturing defect, for one reason or the other. I have no doubt that somebody somewhere sometime had a bolt snap off of a Model 70, but exactly the same is true of a Remington, a Savage, Ruger, and every other brand of rifle.
Enjoy your rifle. There are about hundreds of other things more important to worry about. You have about 1,000 times greater chance getting killed in a car wreck this year, than having your bolt snap off and being injured by a game animal.
And, don't become "mentally invested" in your rifle. It just a piece of machinery,like any other, and a pretty cheap one at that. If you had a $1,000 Snapper lawn mower, I doubt you would be worrying about whether the gear shift hand will break off while your are driving it, resulting in an injury. :-)
Best, Mannyrock