If you go into the clipping dropdown in the curvemapping widget and set change set the min or max to equal each other, the widget disapears.

This patch sets the hardmin and hardmax for those buttons to create a minimum
size for the cliprect on each axis.

This fix is verging on a bit hacky, but it wasn't clear to me that other methods
would be better. Another way to do it would be to validate the rect in
BKE_curvemapping_changed, but that brings up some issues as well.
It seems simpler to do it right where the values are already set.