USB: serial: digi_acceleport: Eliminate a NULL pointer dereference

If port is NULL, then the call to dev_err will dereference a value that is
a small offset from NULL.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@r exists@
expression E,E1;
identifier f;
statement S1,S2,S3;
@@

if ((E == NULL && ...) || ...)
{
  ... when != if (...) S1 else S2
      when != E = E1
* E->f
  ... when any
  return ...;
}
else S3
// </smpl>
Signed-off-by: NJulia Lawall <julia@diku.dk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/digi_acceleport.c

@@ -1239,8 +1239,7 @@ static void digi_write_bulk_callback(struct urb *urb)
 
 	/* port and serial sanity check */
 	if (port == NULL || (priv = usb_get_serial_port_data(port)) == NULL) {
-		dev_err(&port->dev,
-			"%s: port or port->private is NULL, status=%d\n",
+		pr_err("%s: port or port->private is NULL, status=%d\n",
 			__func__, status);
 		return;
 	}