HID: hiddev: protect against disconnect/NULL-dereference race

One of our users reports consistently hitting a NULL dereference that
resolves to the "hid_to_usb_dev(hid);" call in hiddev_ioctl(), when
disconnecting a Lego WeDo USB HID device from an OLPC XO running
Scratch software.  There's a FIXME comment and a guard against the
dereference, but that happens farther down the function than the
initial dereference does.

This patch moves the call to be below the guard, and the user reports
that it fixes the problem for him.  OLPC bug report:
http://dev.laptop.org/ticket/10174

Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/usbhid/hiddev.c

@@ -587,7 +587,7 @@ static long hiddev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	struct hiddev_list *list = file->private_data;
 	struct hiddev *hiddev = list->hiddev;
 	struct hid_device *hid = hiddev->hid;
-	struct usb_device *dev = hid_to_usb_dev(hid);
+	struct usb_device *dev;
 	struct hiddev_collection_info cinfo;
 	struct hiddev_report_info rinfo;
 	struct hiddev_field_info finfo;
@@ -601,9 +601,11 @@ static long hiddev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	/* Called without BKL by compat methods so no BKL taken */
 
 	/* FIXME: Who or what stop this racing with a disconnect ?? */
-	if (!hiddev->exist)
+	if (!hiddev->exist || !hid)
 		return -EIO;
 
+	dev = hid_to_usb_dev(hid);
+
 	switch (cmd) {
 
 	case HIDIOCGVERSION: