Fix T89265: Crash when tabbing through num inputs

Fix by reverting the part of ec30cf0b74
that assigned `but->editval` in `ui_numedit_begin_set_values`.

Causing access freed memory when using tab to switch
to a numeric input and then leaving the textbox by clicking outside.
This was because `ui_numedit_begin_set_values` shouldn't need to set
`but->editval` and overwrite the pointer.
This would set a pointer that had previously been freed,
causing a `NULL` check to fail later on.

Ref D11679

source/blender/editors/interface/interface_handlers.c

@@ -3906,7 +3906,6 @@ static void ui_numedit_begin_set_values(uiBut *but, uiHandleButtonData *data)
   data->startvalue = ui_but_value_get(but);
   data->origvalue = data->startvalue;
   data->value = data->origvalue;
-  but->editval = &data->value;
 }
 
 static void ui_numedit_begin(uiBut *but, uiHandleButtonData *data)
@@ -3935,6 +3934,7 @@ static void ui_numedit_begin(uiBut *but, uiHandleButtonData *data)
   }
   else {
     ui_numedit_begin_set_values(but, data);
+    but->editval = &data->value;
 
     float softmin = but->softmin;
     float softmax = but->softmax;