[media] igorplugusb: fix leaks in error path

Since rc_allocate_device() uses kmalloc, it can returns NULL,
so need to check,  otherwise, NULL derefenrece can happen.

Reported-by: Insu Yun <wuninsu@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>