cfg80211: fix double-free after changing network namespace

cfg80211: fix double-free after changing network namespace

If wdev->wext.keys was initialized it didn't get reset to NULL on
unregister (and it doesn't get set in cfg80211_init_wdev either), but
wdev is reused if unregister was triggered through
cfg80211_switch_netns.

The next unregister (for whatever reason) will try to free
wdev->wext.keys again.

Signed-off-by: Stefan Bühler <source@stbuehler.de>
Link: https://lore.kernel.org/r/20191126100543.782023-1-stefan.buehler@tik.uni-stuttgart.de
Signed-off-by: Johannes Berg <johannes.berg@intel.com>