rsi: Add null check for virtual interfaces in wowlan config

When the "poweroff" command is executed after wowlan enabled, we have
observed a system crash. In the system "poweroff" sequence, network-manager
is sent to inactive state by cleaning up the network interfaces, using
rsi_mac80211_remove_interface() and when driver tries to access those
network interfaces in rsi_wowlan_config() which was invoked by SDIO
shutdown, results in a crash. Added a NULL check before accessing the
network interfaces in rsi_wowlan_config().

Signed-off-by: Sanjay Kumar Konduri <sanjay.konduri@redpinesignals.com>
Signed-off-by: Siva Rebbagondla <siva.rebbagondla@redpinesignals.com>
Signed-off-by: Sushant Kumar Mishra <sushant.mishra@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/rsi/rsi_91x_mac80211.c

@@ -1804,10 +1804,15 @@ int rsi_config_wowlan(struct rsi_hw *adapter, struct cfg80211_wowlan *wowlan)
 	struct rsi_common *common = adapter->priv;
 	u16 triggers = 0;
 	u16 rx_filter_word = 0;
-	struct ieee80211_bss_conf *bss = &adapter->vifs[0]->bss_conf;
+	struct ieee80211_bss_conf *bss = NULL;
 
 	rsi_dbg(INFO_ZONE, "Config WoWLAN to device\n");
 
+	if (!adapter->vifs[0])
+		return -EINVAL;
+
+	bss = &adapter->vifs[0]->bss_conf;
+
 	if (WARN_ON(!wowlan)) {
 		rsi_dbg(ERR_ZONE, "WoW triggers not enabled\n");
 		return -EINVAL;