Make a code path easier for static analyzers to understand

prev cannot be NULL here in the hostapd_eid_country_add() call since
prev is set whenever start becomes non-NULL. That seems to be a bit too
difficult for some static analyzers, so check the prev pointer
explicitly to avoid false warnings.

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

src/ap/beacon.c

@@ -220,7 +220,7 @@ static u8 * hostapd_eid_country(struct hostapd_data *hapd, u8 *eid,
 			continue; /* can use same entry */
 		}
 
-		if (start) {
+		if (start && prev) {
 			pos = hostapd_eid_country_add(pos, end, chan_spacing,
 						      start, prev);
 			start = NULL;