staging: comedi: sanity check num_subdevices parameter in comedi_alloc_subdevices

It's possible for a couple of the comedi drivers to incorrectly call
comedi_alloc_subdevices with num_subdevices = 0. Add a sanity check
before doing the kcalloc.
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbot@mev.co.uk>
Cc: Frank Mori Hess <kmhess@users.sourceforge.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers.c

@@ -60,6 +60,8 @@ int comedi_alloc_subdevices(struct comedi_device *dev, int num_subdevices)
 {
 	int i;
 
+	if (num_subdevices < 1)
+		return -EINVAL;
 	dev->n_subdevices = num_subdevices;
 	dev->subdevices =
 	    kcalloc(num_subdevices, sizeof(struct comedi_subdevice),