Bug fixes #26236 #26237

- Linking logic buttons had draw glitch, caused by reading
  data from free'd buttons
- Doing an undo with mouse-over Curve Widget caused crash or
  corruption.

Both related to code that tries to keep the "active" button in
memory. On each redraw of UI, buttons are re-created, which makes
it all tricky... hope we're safer now.

Also: added back undo pushes on adding links in logic editor.

source/blender/editors/interface/interface_handlers.c

@@ -117,7 +117,6 @@ typedef struct uiHandleButtonData {
 	float vec[3], origvec[3];
 	int togdual, togonly;
 	ColorBand *coba;
-	CurveMapping *cumap;
 
 	/* tooltip */
 	ARegion *tooltip;
@@ -1874,8 +1873,7 @@ static void ui_numedit_begin(uiBut *but, uiHandleButtonData *data)
 	float softrange, softmin, softmax;
 
 	if(but->type == BUT_CURVE) {
-		data->cumap= (CurveMapping*)but->poin;
-		but->editcumap= data->cumap;
+		but->editcumap= (CurveMapping*)but->poin;
 	}
 	else if(but->type == BUT_COLORBAND) {
 		data->coba= (ColorBand*)but->poin;
@@ -3375,7 +3373,7 @@ static int ui_do_but_COLORBAND(bContext *C, uiBlock *block, uiBut *but, uiHandle
 
 static int ui_numedit_but_CURVE(uiBut *but, uiHandleButtonData *data, int snap, int mx, int my)
 {
-	CurveMapping *cumap= data->cumap;
+	CurveMapping *cumap= (CurveMapping*)but->poin;
 	CurveMap *cuma= cumap->cm+cumap->cur;
 	CurveMapPoint *cmp= cuma->curve;
 	float fx, fy, zoomx, zoomy, offsx, offsy;
@@ -3565,7 +3563,7 @@ static int ui_do_but_CURVE(bContext *C, uiBlock *block, uiBut *but, uiHandleButt
 		}
 		else if(event->type==LEFTMOUSE && event->val!=KM_PRESS) {
 			if(data->dragsel != -1) {
-				CurveMapping *cumap= data->cumap;
+				CurveMapping *cumap= (CurveMapping*)but->poin;
 				CurveMap *cuma= cumap->cm+cumap->cur;
 				CurveMapPoint *cmp= cuma->curve;