CVE-2025-68298: Undefined Security Weakness

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

Bluetooth: btusb: mediatek: Avoid btusb_mtk_claim_iso_intf() NULL deref

In btusb_mtk_setup(), we set `btmtk_data->isopkt_intf` to: usb_ifnum_to_if(data->udev, MTK_ISO_IFNUM)

That function can return NULL in some cases. Even when it returns NULL, though, we still go on to call btusb_mtk_claim_iso_intf().

As of commit e9087e828827 ("Bluetooth: btusb: mediatek: Add locks for usb_driver_claim_interface()"), calling btusb_mtk_claim_iso_intf() when `btmtk_data->isopkt_intf` is NULL will cause a crash because we'll end up passing a bad pointer to device_lock(). Prior to that commit we'd pass the NULL pointer directly to usb_driver_claim_interface() which would detect it and return an error, which was handled.

Resolve the crash in btusb_mtk_claim_iso_intf() by adding a NULL check at the start of the function. This makes the code handle a NULL `btmtk_data->isopkt_intf` the same way it did before the problematic commit (just with a slight change to the error message printed).