mac80211: fix memory leaks with element parsing

My previous commit 5d24828d ("mac80211: always allocate
struct ieee802_11_elems") had a few bugs and leaked the new
allocated struct in a few error cases, fix that.

Fixes: 5d24828d

 ("mac80211: always allocate struct ieee802_11_elems")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20211001211108.9839928e42e0.Ib81ca187d3d3af7ed1bfeac2e00d08a4637c8025@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>