media: uvcvideo: Fix access to uninitialized fields on probe error

We need to check whether this work we are canceling actually is
initialized.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Reported-by:  <syzbot+2e1ef9188251d9cc7944@syzkaller.appspotmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>