Dividers are not shortened when inset>0

[phildubach]

On current master (375f59f):

When using a box with an inset and dividers, the dividers extend into the bottom.
I'm using an open box:

TH=2.7; DIA=75; H=90; IN=2;
box(width=DIA*3+4*TH,height=H+TH+IN,depth=DIA*2+3*TH,thickness=TH,
    dividers=[1,2],open=true,assemble=true,spacing=2,inset=IN,kerf=0.1);

This works like a charm, except for the dividers. The inset should shorten them
at the bottom. Sadly, I only noticed after the box was already cut.
See picture. The top of the dividers and side are flush, but the bottom of the
dividers overlaps the finger cutouts for the bottom.

I'll attempt a fix and send a pull request.