[BUG] drivers/tty: NULL pointer dereference in paste_selection()

[co]

We found a bug reachable in:

    path    drivers/tty/vt/selection.c` (fault site), `drivers/accessibility/speakup/main.c` + `drivers/accessibility/speakup
    crash   NULL pointer dereference in paste_selection()
    commit  f5098b6bae76 ("Linux 7.2-rc5")

Config, environment, the sanitizer report and a C reproducer follow.

== Notes ===============================================================
    If you patch the bug based on our artifacts, a tag would be
    appreciated:

Reported-by: co+e5693071d903b832@xxxxxxx

    Everything in this mail is validated by the reproducer below.

    We also hold an LLM-generated root-cause analysis and a candidate
    patch. The patch passes an A/B test: the same reproducer panics the
    unpatched kernel and runs clean on the patched one. Neither has had
    human review, so both still require validation before you send or
    apply them. Available on:

        patch.diff  https://bugs.sh/b/e5693071d903b832/patch.diff
        report.md   https://bugs.sh/b/e5693071d903b832/report.md

    This is an open science project. The code and the full set of PoCs
    are not public at this moment, as we intend to disclose our findings
    in an ethical way.

    Happy to test patches. Complaints and suggestions about our work
    are welcome at:

        cedalion@xxxxxxx

== Environment =========================================================
    Reproduced on  f5098b6bae76 ("Linux 7.2-rc5")
    VM setup       https://bugs.sh/b/e5693071d903b832/run.sh
    config         https://bugs.sh/b/e5693071d903b832/config.gz
    poc            https://bugs.sh/b/e5693071d903b832/repro.c

== Sanitizer Report ====================================================
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000044: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000220-0x0000000000000227]
CPU: 1 UID: 0 PID: 49 Comm: kworker/1:1 Not tainted 7.2.0-rc5 #10 PREEMPTLAZY
Workqueue: events __speakup_paste_selection
RIP: 0010:paste_selection (drivers/tty/vt/selection.c:393)
Call Trace:
__speakup_paste_selection (drivers/accessibility/speakup/selection.c:112)
process_one_work (kernel/workqueue.c:3322)
worker_thread (kernel/workqueue.c:3405 kernel/workqueue.c:3486)
kthread (kernel/kthread.c:436)
ret_from_fork (arch/x86/kernel/process.c:158)
ret_from_fork_asm (arch/x86/entry/entry_64.S:245)
Kernel panic - not syncing: Fatal exception


---
The report format is based on syzbot bug report.

This report is generated by a bot. It may contain errors.
See https://github.com/n132/cedalion for more information.

For any issue with this report, reach out to cedalion@xxxxxxx

If the report is already addressed, let us know by replying with:
#co fix: <commit hash>

If the report is a duplicate of another one, reply with:
#co dup: <lore link>

If you want to undo deduplication, reply with:
#co undup