usb: core: urb: fix URB structure initialization function

Explicitly initialize URB structure urb_list field in usb_init_urb().
This field can be potentially accessed uninitialized and its
initialization is coherent with the usage of list_del_init() in
usb_hcd_unlink_urb_from_ep() and usb_giveback_urb_bh() and its
explicit initialization in usb_hcd_submit_urb() error path.

Signed-off-by: Emiliano Ingrassia <ingrassia@epigenesys.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191127160355.GA27196@ingrassia.epigenesys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>