src/ssd/ssd_part.c: ensure button is found

src/ssd/ssd_part.c: ensure button is found

This fixes an issue detected by the static analyzer.
It currently is not a real bug but it could become
one in the future if `add_toggled_icon()` is called
before `add_scene_button()`.

Rather than having `add_toggled_icon()` go through
the list, find the root button and then fetch its
node descriptor just supply the button as argument.