Can I do anything to protect myself
If possible bring it out to the open. Ask the person who broke the rule to share what they've done and why in a team channel rather than in a direct message to you. This not only increases transparency and accountability in the team (those are good things, right?) but may also help take care of this:
am I basically screwed if this person decides to make wild accusations?
Since it was them who brought it out in the open - not you - you're in the clear.
Use the argument that having a conversation about this in the open (e.g. in a team channel or in email thread) will help the entire team deal with similar situations in the future, and it's employee's duty to do what's right. It also takes some of the culpability off the perpetrator - since they were transparent about it.
If you have legitimate concerns of retaliation - share those with the manager, especially if there were prior incidents of such behavior.
If (1) you aren't sure of manager's fairness and susceptibility to frivolous, untrue retaliation claims, and (2) would rather not put your employment in jeopardy - do not report the incident unless that exposes you to liability.
P.S. I do not find this (from the other answer) to be correct:
Make sure you haven't done anything that could be used against you, and you have nothing to worry about.
Many a person has been fired or retaliated against on false claims, and in some companies - probably quite a few - your performance matters little vs. who you know and who has your back.