Again. The chance of that is nigh zero. The only one who might buy into this story enough to send an assassin is Cersei and she has not the resources to do so without Robert or Tywin indulging her, which they won't.
The most that will come of this is that Varys replacement will look into the matter.
And even if it does happen, we can ressurect Stannis.
So a astronomically low change of Stannis getting killed by Cersei versus a extremely high chance of us having to kill Stannis on the battlefield or to cement the peace.
This plan is much better for his long-term survival.
And honestly, ever since we got informed that Stannis might go back on the Small Council, I was looking for ways to prevent this to keep him neutral. I couldn't come up with anything aside from framing him for something or asking him to turn the position down, the latter of the two options being almost guaranteed to fail.