is broth that hasn't been boiled safe?
170 degrees is perfectly fine. 

To talk bacteria, you have to take into account two factors: temperature, and time. Anything between 40 and 140 is good for them, anything above 140 is bad. At the same time, food in the dangerzone that is eaten/cooked/frozen promptly, is fine, because it takes time to build up a colony of harmful proportions.

In this case your temperature is 170 degrees, which is hot enough to kill most common bacteria instantly (Milk is pasteurized in the 160s), and then you keep it at that temperature for hours? The toughest common bacteria (C. botulinum) dies very quickly at 180 degrees...It's the only one I know off the top of my head that doesn't immediately die in the 170 range. And keeping your stock at 170 for 10 minutes or so will kill any C. botulinum that may have somehow found it's way inside it.

You've got nothing to worry about.

(Basics of TDT calculation)