GHSA-q56x-g2fj-4rj6: ONNX: TOCTOU arbitrary file read/write in save_external_dat

ONNX's `save_external_data` method contains a TOCTOU vulnerability (time-of-check-time-of-use, a gap between checking if a file exists and using it) that allows attackers to overwrite arbitrary files by creating symlinks (shortcuts to other files) between those two operations. The code also has a potential path validation bypass on Windows systems that may allow absolute paths to be used.