mac80211: fix possible memory leak in ieee80211_assign_beacon

mac80211: fix possible memory leak in ieee80211_assign_beacon

Free new beacon_data in ieee80211_assign_beacon whenever
ieee80211_assign_beacon fails

Fixes: 8860020e0be1 ("cfg80211: restructure AP/GO mode API")
Fixes: bc847970f432 ("mac80211: support FTM responder configuration/statistic")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/770285772543c9fca33777bb4ad4760239e56256.1562105631.git.lorenzo@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>