mac80211: fix possible sta leak

If TDLS station addition is rejected, the sta memory is leaked.
Avoid this by moving the check before the allocation.

Cc: stable@vger.kernel.org
Fixes: 7ed52853

 ("mac80211: don't initiate TDLS connection if station is not associated to AP")
Link: https://lore.kernel.org/r/20190801073033.7892-1-johannes@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>