b43: Fix swatch warning

Swatch reports the following warning for main.c:

  CHECK   drivers/net/wireless/b43/main.c
drivers/net/wireless/b43/main.c +4115 b43_wireless_core_stop(7) warn: variable dereferenced before check 'dev'

After analysis, this is not a bug, but a false warning. Nonetheless,
a cleanup is in order to prevent some future janitor proposing
the wrong fix, as I did in my original patch.
Signed-off-by: NLarry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/b43/main.c

@@ -4131,10 +4131,13 @@ static void b43_op_configure_filter(struct ieee80211_hw *hw,
  * because the core might be gone away while we unlocked the mutex. */
 static struct b43_wldev * b43_wireless_core_stop(struct b43_wldev *dev)
 {
-	struct b43_wl *wl = dev->wl;
+	struct b43_wl *wl;
 	struct b43_wldev *orig_dev;
 	u32 mask;
 
+	if (!dev)
+		return NULL;
+	wl = dev->wl;
 redo:
 	if (!dev || b43_status(dev) < B43_STAT_STARTED)
 		return dev;