Hi Cedric,
Thanks for filing this bug report with the recipe.
While I saw a crash happen once, I wasn't quite sure what caused that and couldn't come up with a recipe.
But I am pretty sure that I came across the same bug.. I had happened to slightly scroll the mouse wheel unknowingly while I was about to click on the emacs frame and emacs crashed.
@Martin: Thanks for the fix! I confirm the bug presence as well as the fix after rebuilding emacs using commit fc05d4f.