mac80211: add missing null return check from call to ieee80211_get_sband

The return from ieee80211_get_sband can potentially be a null pointer, so
it seems prudent to add a null check to avoid a null pointer dereference
on sband.

Addresses-Coverity: ("Dereference null return")
Fixes: 2ab45876 ("mac80211: add support for the ADDBA extension element")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20190730143205.14261-1-colin.king@canonical.comSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/agg-rx.c

@@ -189,6 +189,8 @@ static void ieee80211_add_addbaext(struct ieee80211_sub_if_data *sdata,
 	u8 *pos;
 
 	sband = ieee80211_get_sband(sdata);
+	if (!sband)
+		return;
 	he_cap = ieee80211_get_he_iftype_cap(sband, sdata->vif.type);
 	if (!he_cap)
 		return;