prevent crash when exporting spatial bookmarks (fix #16345)

(cherry picked from commit 58a8478)

src/app/qgsbookmarks.cpp

@@ -348,7 +348,7 @@ void QgsBookmarks::exportToXML()
   doc.appendChild( root );
 
   int rowCount = mModel->rowCount();
-  int colCount = mModel->columnCount();
+  int colCount = mModel->columnCount() - 1;  // exclude virtual "In project" column
 
   QList<QString> headerList;
   headerList << "id" << "name" << "project" << "xmin" << "ymin" << "xmax" << "ymax" << "sr_id";