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: NArvind Yadav <arvind.yadav.cs@gmail.com>
Tested-by: NAndrey Konovalov <andreyknvl@google.com>
Signed-off-by: NSean Young <sean@mess.org>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/rc/imon.c

@@ -2515,6 +2515,11 @@ static int imon_probe(struct usb_interface *interface,
 	mutex_lock(&driver_lock);
 
 	first_if = usb_ifnum_to_if(usbdev, 0);
+	if (!first_if) {
+		ret = -ENODEV;
+		goto fail;
+	}
+
 	first_if_ctx = usb_get_intfdata(first_if);
 
 	if (ifnum == 0) {