Short answer: No.
With this reasoning no upgrade or update will ever be safe to do for you.
Just stay on Android 5 getting no updates any more and wait for somebody to remotely tamper with your phone via some security hole that was patched with a later upgrade or update. Currently you want to google “BlueBorne”.
There is no such thing as a bug-free OS. But with every update there is at least hope that the bugs are getting less … until there’s a version upgrade of course, and the circle starts anew …