I would guess, seeing as this hasn't happened to you, that your technique is probably fine. It is a fairly rare injury, but is one which makes a very memorable image, which is probably why you find yourself thinking about it. I imagine that when it has happened it's been the result of an accidental slip, which meant there was a lot of force focused on that one body part, but which could have just as easilly been a concussion or twisted ankle.
I would file this with other unlikely injuries which we can't think about constantly, beyond basic risk reduction. We can try and pay attention and be quick-footed, but who knows, a piano could still fall on my head!
Being sober during sex also helps. Many sexual mishaps - all that I've heared about - seem to have happened when people's physical senses and/or attention and/or wisdom has been blunted by alcohol or whatever else.
I'd say this is somehting worth talking to your girlfriend about too. That could reduce the fear but could also help you both to take care and pay attention to eachothers discomfort during sex.
"In between two tall mountains there's a place they call lonesome.
Don't see why they call it lonesome.
I'm never lonesome when I go there." Connie Converse - Talkin' Like You