HID: intel-ish-hid: fix NULL-ptr-deref in ishtp_bus_remove_all_clients

During a warm reset flow, the cl->device pointer may be NULL if the
reset occurs while clients are still being enumerated. Accessing
cl->device->reference_count without a NULL check leads to a kernel panic.

This issue was identified during multi-unit warm reboot stress clycles.
Add a defensive NULL check for cl->device to ensure stability under
such intensive testing conditions.

KASAN: null-ptr-deref in range [0000000000000000-0000000000000007]
Workqueue: ish_fw_update_wq fw_reset_work_fn

Call Trace:
 ishtp_bus_remove_all_clients+0xbe/0x130 [intel_ishtp]
 ishtp_reset_handler+0x85/0x1a0 [intel_ishtp]
 fw_reset_work_fn+0x8a/0xc0 [intel_ish_ipc]

Fixes: 3703f53 ("HID: intel_ish-hid: ISH Transport layer")
Signed-off-by: Ryan Lin <ryan.lin@intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>

Author: ryanlin0317

drivers/hid/intel-ish-hid/ishtp/bus.c

@@ -730,7 +730,7 @@ void ishtp_bus_remove_all_clients(struct ishtp_device *ishtp_dev,
 	spin_lock_irqsave(&ishtp_dev->cl_list_lock, flags);
 	list_for_each_entry(cl, &ishtp_dev->cl_list, link) {
 		cl->state = ISHTP_CL_DISCONNECTED;
-		if (warm_reset && cl->device->reference_count)
+		if (warm_reset && cl->device && cl->device->reference_count)
 			continue;
 
 		/*