media: dvb-usb/dvb-usb-urb.c: initialize actlen to 0

commit 569bc8d6 upstream.

This fixes a syzbot failure since actlen could be uninitialized,
but it was still used.

Syzbot link:

https://syzkaller.appspot.com/bug?extid=6bf9606ee955b646c0e1



Reported-and-tested-by:  <syzbot+6bf9606ee955b646c0e1@syzkaller.appspotmail.com>

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Acked-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>