isdn: mISDN: hfcsusb: Fix possible null-pointer dereferences in start_isoc_chain()

In start_isoc_chain(), usb_alloc_urb() on line 1392 may fail
and return NULL. At this time, fifo->iso[i].urb is assigned to NULL.

Then, fifo->iso[i].urb is used at some places, such as:
LINE 1405:    fill_isoc_urb(fifo->iso[i].urb, ...)
                  urb->number_of_packets = num_packets;
                  urb->transfer_flags = URB_ISO_ASAP;
                  urb->actual_length = 0;
                  urb->interval = interval;
LINE 1416:    fifo->iso[i].urb->...
LINE 1419:    fifo->iso[i].urb->...

Thus, possible null-pointer dereferences may occur.

To fix these bugs, "continue" is added to avoid using fifo->iso[i].urb
when it is NULL.

These bugs are found by a static analysis tool STCheck written by us.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/hardware/mISDN/hfcsusb.c

@@ -1394,6 +1394,7 @@ start_isoc_chain(struct usb_fifo *fifo, int num_packets_per_urb,
 				printk(KERN_DEBUG
 				       "%s: %s: alloc urb for fifo %i failed",
 				       hw->name, __func__, fifo->fifonum);
+				continue;
 			}
 			fifo->iso[i].owner_fifo = (struct usb_fifo *) fifo;
 			fifo->iso[i].indx = i;