Thank you for your post. i think you should book a taxi from trusted travel agent well in advance before coming to Patna airport so that you can reach your destination without fear. Thanks ...
you are in misinformed , there may be couple of incidents decades ago. All people of the region are living peacefully and there are thousands of tourists travel by road on daily basis.
you can book taxi/cab by trusted agency like EazeeCab, KTEC, Naland tours or others and reach the Bodhgaya safely before night. travel time is 2.5hrs to 3 hrs so you will be in Bodhgaya before night.
Like every new place you need to care few things you may consider same for Bodhgaya but nothing to worry much you will find thousands of tourists in the town.
have good travel