ubi: Fix possible null-ptr-deref in ubi_free_volume()

ubi: Fix possible null-ptr-deref in ubi_free_volume()

It willl cause null-ptr-deref in the following case:

uif_init()
  ubi_add_volume()
    cdev_add() -> if it fails, call kill_volumes()
    device_register()

kill_volumes() -> if ubi_add_volume() fails call this function
  ubi_free_volume()
    cdev_del()
    device_unregister() -> trying to delete a not added device,
			   it causes null-ptr-deref

So in ubi_free_volume(), it delete devices whether they are added
or not, it will causes null-ptr-deref.

Handle the error case whlie calling ubi_add_volume() to fix this
problem. If add volume fails, set the corresponding vol to null,
so it can not be accessed in kill_volumes() and release the
resource in ubi_add_volume() error path.

Fixes: 801c135ce73d ("UBI: Unsorted Block Images")
Suggested-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>