kmscon crashes with SIGSEGV when a udev "change" (hotplug) event arrives for
a DRM device that failed to initialize. The crash occurs because neither
seat_monitor_hotplug_dev() nor kmscon_seat_poll_video() check for NULL
dev_data before dereferencing it.

When a DRM device fails seat_video_init() during the initial scan,
kmscon_seat_add_video() correctly sets dev->data to NULL and frees the
kmscon_video struct. The device remains in the monitor's device list. When
udev subsequently sends a "change" event for that device (e.g., HDMI hotplug),
monitor_udev_change() calls:

    sdev->mon->cb.hotplug_dev(sdev->mon->data, sdev->type, sdev->data);

with sdev->data == NULL, which reaches:

    static void seat_monitor_hotplug_dev(..., void *dev_data)
    {
        if (type == UTERM_MONITOR_DRM || type == UTERM_MONITOR_FBDEV)
            kmscon_seat_poll_video(dev_data);  // dev_data is NULL
    }

    static void kmscon_seat_poll_video(void *data)
    {
        struct kmscon_video *vid = data;  // NULL
        video_poll(vid->video);           // SIGSEGV
    }

This is easily reproducible on Raspberry Pi 4 which has a V3D render-only
DRM device (/dev/dri/card1) that lacks dumb buffer support, causing
seat_video_init() to fail with -EOPNOTSUPP.

Version-Release number of selected component:
kmscon-10.0.2-2.fc45.aarch64


Reproducible: Always

Steps to Reproduce:
1. Boot aarch Xfce Fedora 45 Branched 20260826.n.0 on Raspberry Pi 4 Model B
2. kmscon starts (either kmsconvt@tty1 or via initial-setup.service)
3. kmscon fails to init /dev/dri/card1 (V3D, no dumb buffer support)
4. udev sends a "change" event for card1 (e.g., hotplug notification)
5. kmscon crashes with SIGSEGV in monitor_udev_event

Actual Results:
kmscon crashes with SIGSEGV in monitor_udev_event preventing whatever was running in it from completion

Expected Results:
kmscon does not crash

Additional Information:
Crash stack trace:
#0  monitor_udev_event (kmscon + 0x192ec)
#1  ev_eloop_dispatch (kmscon + 0xd318)
#2  main (kmscon + 0x9fac)

Relevant journal output:
kmscon: ERROR: video_drm2d: driver does not support dumb buffers
kmscon: ERROR: seat: cannot create video device /dev/dri/card1 on seat seat0: -95

Suggested fix in src/seat.c:

1. Add NULL check in seat_monitor_hotplug_dev():

    if (type == UTERM_MONITOR_DRM || type == UTERM_MONITOR_FBDEV)
   -    kmscon_seat_poll_video(dev_data);
   +    if (dev_data)
   +        kmscon_seat_poll_video(dev_data);

2. Add NULL check in kmscon_seat_poll_video() as defense-in-depth:

    struct kmscon_video *vid = data;
   +if (!vid)
   +    return;
    log_debug("poll video device %s on seat %s", vid->node, vid->seat->name);
    video_poll(vid->video);