mac80211: Fix null dereference in ieee80211_key_link()

mac80211: Fix null dereference in ieee80211_key_link()

Commit ef810e7c3d2a ("mac80211: accept key reinstall without changing
anything") moved the initialisation of key->sdata later in
ieee80211_key_link().  In the upstream commit fdf7cb4185b6 this was
fine, but in this version of the function there is additional code
which relies on key->sdata.  Change this to use the value that will be
(conditionally) assigned to it later.

Cc: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>