WFD: Clear wfd_subelems when P2P peer stops sending them

When a peer device stops sending wfd_subelems, wpa_supplicant should
remove dev->info.wfd_subelems from peer's properties. Previously,
wpa_supplicant left the previously learned dev->info.wfd_subelems in
place whenever the new message did not include wfd_subelems.

In addition to fixing the clearing of the old wfd_subelems, this
resolves another issue. As "wfd_changed" variable becomes true even when
peer stops sending wfd_subelems and dev->info.wfd_subelems has an old
value, a new P2P-DEVICE-FOUND event notification was sent again and
again to upper layers whenever a new discovery response was received
from the peer that previously advertised WFD subelements.

Signed-off-by: Mayank Haarit <mayank.h@samsung.com>

src/p2p/p2p.c

@@ -839,9 +839,12 @@ int p2p_add_device(struct p2p_data *p2p, const u8 *addr, int freq,
 
 	wfd_changed = p2p_compare_wfd_info(dev, &msg);
 
-	if (msg.wfd_subelems) {
+	if (wfd_changed) {
 		wpabuf_free(dev->info.wfd_subelems);
-		dev->info.wfd_subelems = wpabuf_dup(msg.wfd_subelems);
+		if (msg.wfd_subelems)
+			dev->info.wfd_subelems = wpabuf_dup(msg.wfd_subelems);
+		else
+			dev->info.wfd_subelems = NULL;
 	}
 
 	if (scan_res) {