The xobjs variable was used outside the try-except block that defined it.
If a KeyError was caught and the id started/ended with '~' (inline images),
the code would continue but xobjs would remain undefined, causing
UnboundLocalError when trying to access non-inline images.

Initialize xobjs to None and check before using it.

---------

Co-authored-by: Yuki9814 <Yuki9814@users.noreply.github.com>
Co-authored-by: Yuki9814 <222397878+Yuki9814@users.noreply.github.com>

This only affects the non-strict mode and is triggered for errors which
should always trigger a hard error, id est recursion and filter limit
errors.

## What's new

### Security (SEC)
- Avoid infinite loop in read_from_stream for broken files (#3693) by @stefan6419846

### Robustness (ROB)
- Resolve UnboundLocalError for xobjs in _get_image (#3684) by @Yuki9814

[Full Changelog](6.9.1...6.9.2)