ieee1394: fix NULL pointer dereference in sysfs access

Regression since "ieee1394: prevent device binding of raw1394,
video1394, dv1394", commit d2ace29fa44589da51fedc06a67b3f05301f3bfd:
$ cat /sys/bus/ieee1394/drivers/raw1394/device_ids
triggers a NULL pointer dereference in fw_show_drv_device_ids.
Reported by Miles Lane.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Tested-by: Miles Lane <miles.lane@gmail.com>

drivers/ieee1394/nodemgr.c

@@ -520,8 +520,11 @@ static ssize_t fw_show_drv_device_ids(struct device_driver *drv, char *buf)
 	char *scratch = buf;
 
 	driver = container_of(drv, struct hpsb_protocol_driver, driver);
+	id = driver->id_table;
+	if (!id)
+		return 0;
 
-	for (id = driver->id_table; id->match_flags != 0; id++) {
+	for (; id->match_flags != 0; id++) {
 		int need_coma = 0;
 
 		if (id->match_flags & IEEE1394_MATCH_VENDOR_ID) {