Fix (unreported) invalid memory access in new 'newer blendfile version' code.

When choosing the new 'overwrite' option when trying to save a blendfile
from a newer version of Blender, it would cause invalid (use-after-free)
memory access.

Issue caused by the main commit (a1d7ec7139) of the new blendfile
compatibility handling. No idea how it was not detected earlier.

Many thanks to @weizhen for spotting the issue and doing some initial
investigation on it.

source/blender/windowmanager/intern/wm_files.cc

@@ -3830,12 +3830,16 @@ static void save_file_forwardcompat_cancel_button(uiBlock *block, wmGenericCallb
 static void save_file_forwardcompat_overwrite(bContext *C, void *arg_block, void *arg_data)
 {
   wmWindow *win = CTX_wm_window(C);
-  UI_popup_block_close(C, win, static_cast<uiBlock *>(arg_block));
 
   /* Re-use operator properties as defined for the initial 'save' operator, which triggered this
    * 'forward compat' popup. */
   wmGenericCallback *callback = WM_generic_callback_steal(
       static_cast<wmGenericCallback *>(arg_data));
+
+  /* Needs to be done after stealing the callback data above, otherwise it would cause a
+   * use-after-free. */
+  UI_popup_block_close(C, win, static_cast<uiBlock *>(arg_block));
+
   PointerRNA operator_propptr = {};
   PointerRNA *operator_propptr_p = &operator_propptr;
   IDProperty *operator_idproperties = static_cast<IDProperty *>(callback->user_data);