staging: comedi: jr3_pci: fix possible null pointer dereference

For some reason, the driver does not consider allocation of the
subdevice private data to be a fatal error when attaching the COMEDI
device.  It tests the subdevice private data pointer for validity at
certain points, but omits some crucial tests.  In particular,
`jr3_pci_auto_attach()` calls `jr3_pci_alloc_spriv()` to allocate and
initialize the subdevice private data, but the same function
subsequently dereferences the pointer to access the `next_time_min` and
`next_time_max` members without checking it first.  The other missing
test is in the timer expiry routine `jr3_pci_poll_dev()`, but it will
crash before it gets that far.

Fix the bug by returning `-ENOMEM` from `jr3_pci_auto_attach()` as soon
as one of the calls to `jr3_pci_alloc_spriv()` returns `NULL`.  The
COMEDI core will subsequently call `jr3_pci_detach()` to clean up.

Cc: <stable@vger.kernel.org> # 3.15+
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/jr3_pci.c

@@ -727,11 +727,12 @@ static int jr3_pci_auto_attach(struct comedi_device *dev,
 		s->insn_read	= jr3_pci_ai_insn_read;
 
 		spriv = jr3_pci_alloc_spriv(dev, s);
-		if (spriv) {
-			/* Channel specific range and maxdata */
-			s->range_table_list	= spriv->range_table_list;
-			s->maxdata_list		= spriv->maxdata_list;
-		}
+		if (!spriv)
+			return -ENOMEM;
+
+		/* Channel specific range and maxdata */
+		s->range_table_list	= spriv->range_table_list;
+		s->maxdata_list		= spriv->maxdata_list;
 	}
 
 	/* Reset DSP card */