mac80211: remove unnecessary NULL check

Both old and new cannot be NULL at the same time, hence checking
new when old is not NULL is unnecessary.

Also, notice that new is being dereferenced before it is checked:

	idx = new->conf.keyidx;

The above triggers a static code analysis warning.

Address this by removing the NULL check on new and adding a code
comment based on the following piece of code:

387        /* caller must provide at least one old/new */
388        if (WARN_ON(!new && !old))
389                return 0;

Addresses-Coverity-ID: 1473176 ("Dereference before null check")
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>

net/mac80211/key.c

@@ -401,8 +401,9 @@ static int ieee80211_key_replace(struct ieee80211_sub_if_data *sdata,
 		 * pairwise keys.*/
 		ret = ieee80211_hw_key_replace(old, new, pairwise);
 	} else {
+		/* new must be provided in case old is not */
 		idx = new->conf.keyidx;
-		if (new && !new->local->wowlan)
+		if (!new->local->wowlan)
 			ret = ieee80211_key_enable_hw_accel(new);
 		else
 			ret = 0;