Fix T43111: Node Editor (Slider) Draw Glitch

* don't allow Node Editor input max value to be less then min value
* avoid the num slider drawing glitch if softmin equals softmax
* assert if softmax/hardmax is smaller than softmin/hardmin

With this, we sort of allow softmin/hardmin and softmax/hardmax being the same.

source/blender/editors/interface/interface.c

@@ -2603,6 +2603,10 @@ void ui_but_update(uiBut *but)
 		}
 	}
 
+	/* max must never be smaller than min! Both being equal is allowed though */
+	BLI_assert(but->softmin <= but->softmax &&
+	           but->hardmin <= but->hardmax);
+
 	/* test for min and max, icon sliders, etc */
 	switch (but->type) {
 		case UI_BTYPE_NUM: