wext: fix NULL-ptr-dereference with cfg80211's lack of commit()

Since cfg80211 doesn't implement commit, we never really cared about
that code there (and it's configured out w/o CONFIG_WIRELESS_EXT).
After all, since it has no commit, it shouldn't return -EIWCOMMIT to
indicate commit is needed.

However, EIWCOMMIT is actually an alias for EINPROGRESS, which _can_
happen if e.g. we try to change the frequency but we're already in
the process of connecting to some network, and drivers could return
that value (or even cfg80211 itself might).

This then causes us to crash because dev->wireless_handlers is NULL
but we try to check dev->wireless_handlers->standard[0].

Fix this by also checking dev->wireless_handlers. Also simplify the
code a little bit.

Cc: stable@vger.kernel.org
Reported-by: syzbot+444248c79e117bc99f46@syzkaller.appspotmail.com
Reported-by: syzbot+8b2a88a09653d4084179@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/20210121171621.2076e4a37d5a.I5d9c72220fe7bb133fb718751da0180a57ecba4e@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/wext-core.c

@@ -896,8 +896,9 @@ static int ioctl_standard_iw_point(struct iw_point *iwp, unsigned int cmd,
 int call_commit_handler(struct net_device *dev)
 {
 #ifdef CONFIG_WIRELESS_EXT
-	if ((netif_running(dev)) &&
-	   (dev->wireless_handlers->standard[0] != NULL))
+	if (netif_running(dev) &&
+	    dev->wireless_handlers &&
+	    dev->wireless_handlers->standard[0])
 		/* Call the commit handler on the driver */
 		return dev->wireless_handlers->standard[0](dev, NULL,
 							   NULL, NULL);