mac80211: verify ieee80211_key_replace() arguments

There's no code calling ieee80211_key_replace() with both
arguments NULL and it wouldn't make sense, but in the
interest of maintainability add a warning for it. As a
side effect, this also shuts up a smatch warning.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/key.c

@@ -260,6 +260,10 @@ static void ieee80211_key_replace(struct ieee80211_sub_if_data *sdata,
 	int idx;
 	bool defunikey, defmultikey, defmgmtkey;
 
+	/* caller must provide at least one old/new */
+	if (WARN_ON(!new && !old))
+		return;
+
 	if (new)
 		list_add_tail(&new->list, &sdata->key_list);