media: pvrusb2: fix null-ptr-deref in class_unregister()

The class_ptr will be NULL if pvr2_sysfs_class_create() fails
in pvr_init(), when call pvr2_sysfs_class_destroy(), it will
lead to null-ptr-deref, fix it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>