usb: initialize libusb_device to avoid crash

If libusb_get_device_list() fails, the uninitialized local variable
libusb_device would be passed to libusb_free_device_list(), that
will cause a crash, like:
(gdb) bt
 #0  0x00007fbbb4bafc10 in pthread_mutex_lock () from /lib64/libpthread.so.0
 #1  0x00007fbbb233e653 in libusb_unref_device (dev=0x6275682d627375)
     at core.c:902
 #2  0x00007fbbb233e739 in libusb_free_device_list (list=0x7fbbb6e8436e,
     unref_devices=<optimized out>) at core.c:653
 #3  0x00007fbbb6cd80a4 in usb_host_auto_check (unused=unused@entry=0x0)
     at hw/usb/host-libusb.c:1446
 #4  0x00007fbbb6cd8525 in usb_host_initfn (udev=0x7fbbbd3c5670)
     at hw/usb/host-libusb.c:912
 #5  0x00007fbbb6cc123b in usb_device_init (dev=0x7fbbbd3c5670)
     at hw/usb/bus.c:106
 ...

So initialize libusb_device at the begin time.

Signed-off-by: Jincheng Miao <jmiao@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
