Staging: comedi: fix bug with invalid minor number usage.

Return error instead of segfaulting if user trys to run comedi_config on
a device file with and invalid minor number.

From: Frank Mori Hess <fmhess@users.sourceforge.net>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: David Schleef <ds@schleef.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/comedi/comedi_fops.c

@@ -104,9 +104,13 @@ static int comedi_ioctl(struct inode *inode, struct file *file,
 	const unsigned minor = iminor(file->f_dentry->d_inode);
 	struct comedi_device_file_info *dev_file_info =
 	    comedi_get_device_file_info(minor);
-	comedi_device *dev = dev_file_info->device;
+	comedi_device *dev;
 	int rc;
 
+	if (dev_file_info == NULL || dev_file_info->device == NULL)
+		return -ENODEV;
+	dev = dev_file_info->device;
+
 	mutex_lock(&dev->mutex);
 
 	/* Device config is special, because it must work on