Is a 12V voltage step-up USB to 4-Pin PC adapter safe to use in an enclosed space for extended periods of time?
I think this one would be fine.  Like any new purchase, don't trust it alone until you've babysat it a little while, but assuming it's not defective you should be safe...you're not even getting close to capacity with the 53mA your fan is using.  It's designed for 400mA.