HID: logitech-dj: Fix maxfield check in DJ short report validation

In the Linux kernel, the following vulnerability has been resolved:

HID: logitech-dj: Fix maxfield check in DJ short report validation

Commit b6a57912854e ("HID: logitech-dj: Prevent REPORT_ID_DJ_SHORT
related user initiated OOB write") added validation for the DJ short
output report, but the error path dereferences rep->field[0] even when
rep->maxfield is zero.

Commit 8b9a097eb2fc ("HID: logitech-dj: fix wrong detection of bad
DJ_SHORT output report") made the check conditional on rep being present,
but a crafted descriptor can still create report ID 0x20 with only padding
output items. hid-core registers the report, ignores the padding field,
and leaves rep->maxfield as zero.

In that case the validation enters the rep->maxfield < 1 branch and then
dereferences rep->field[0]->report_count while printing the error message,
causing a NULL pointer dereference during probe. This is reproducible with
uhid by emulating a Logitech receiver with a padding-only DJ short output
report:

BUG: KASAN: null-ptr-deref in logi_dj_probe+0xb1/0x754 [hid_logitech_dj]
Read of size 4 at addr 0000000000000028 by task kworker/4:1/129
...
Call Trace:
logi_dj_probe+0xb1/0x754 [hid_logitech_dj]
hid_device_probe+0x329/0x3f0 [hid]
really_probe+0x162/0x570
__device_attach+0x137/0x2c0
bus_probe_device+0x38/0xc0
device_add+0xa56/0xce0
hid_add_device+0x19c/0x280 [hid]
uhid_device_add_worker+0x2c/0xb0 [uhid]

Reject the zero-field report before printing the field report_count.