[app] Ensure calculated histogram spike height ...

is positive or zero. Sometimes the calculated `y` or `bg_y`
values are negative because the histogram value was divided
by zero (when max == 0.0).

Resolves: #9132 (Gnome/GIMP bug tracker)
(cherry picked from commit 9b517fbf7d)

app/widgets/gimphistogramview.c

@@ -500,6 +500,9 @@ gimp_histogram_view_draw_spike (GimpHistogramView    *view,
       break;
     }
 
+  y    = MAX (y,    0);
+  bg_y = MAX (bg_y, 0);
+
   if (bg_color)
     {
       gdk_cairo_set_source_color (cr, bg_color);