media: imon: Fix null-ptr-deref in imon_probe

It seems that the return value of usb_ifnum_to_if() can be NULL and
needs to be checked.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>