staging: comedi: addi_apci_3xxx: check return value

Function pci_ioremap_bar() will return a NULL pointer if there is no
enough memory. However, in function apci3xxx_auto_attach(), the return
value of function pci_ioremap_bar() is not validated. This may result in
NULL dereference in following access to dev->mmio. This patch fixes the
bug.
Signed-off-by: Pan Bian <bianpan2016@163.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/addi_apci_3xxx.c

@@ -787,6 +787,8 @@ static int apci3xxx_auto_attach(struct comedi_device *dev,
 
 	dev->iobase = pci_resource_start(pcidev, 2);
 	dev->mmio = pci_ioremap_bar(pcidev, 3);
+	if (!dev->mmio)
+		return -ENOMEM;
 
 	if (pcidev->irq > 0) {
 		ret = request_irq(pcidev->irq, apci3xxx_irq_handler,