AP: Avoid undefined behavior in pointer arithmetic in IE parsing

Reorder terms in a way that no invalid pointers are generated with
pos+len operations. end-pos is always defined (with a valid pos pointer)
while pos+len could end up pointing beyond the end pointer which would
be undefined behavior.

Signed-off-by: Jouni Malinen <j@w1.fi>

src/ap/beacon.c

@@ -553,8 +553,8 @@ static enum ssid_match_result ssid_match(struct hostapd_data *hapd,
 
 	pos = ssid_list;
 	end = ssid_list + ssid_list_len;
-	while (pos + 1 <= end) {
-		if (pos + 2 + pos[1] > end)
+	while (end - pos >= 1) {
+		if (2 + pos[1] > end - pos)
 			break;
 		if (pos[1] == 0)
 			wildcard = 1;