HID: hidraw, fix a NULL pointer dereference in hidraw_ioctl

BUG: unable to handle kernel NULL pointer dereference at 0000000000000028
IP: [<ffffffffa02c66b4>] hidraw_ioctl+0xfc/0x32c [hid]
[...]

This is reproducible by disconnecting the device while userspace does
ioctl in a loop and doesn't check return values in order to exit the
loop.
Signed-off-by: NAntonio Ospite <ospite@studenti.unina.it>
Cc: stable@kernel.org
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

drivers/hid/hidraw.c

@@ -244,6 +244,10 @@ static long hidraw_ioctl(struct file *file, unsigned int cmd,
 
 	mutex_lock(&minors_lock);
 	dev = hidraw_table[minor];
+	if (!dev) {
+		ret = -ENODEV;
+		goto out;
+	}
 
 	switch (cmd) {
 		case HIDIOCGRDESCSIZE:
@@ -317,6 +321,7 @@ static long hidraw_ioctl(struct file *file, unsigned int cmd,
 
 		ret = -ENOTTY;
 	}
+out:
 	mutex_unlock(&minors_lock);
 	return ret;
 }