Interworking: Make bounds checking easier for static analyzers

'num * 5 > end - pos' handles bounds checking a bit more efficiently,
but apparently that is not clear enough for all static analyzers.
Replace with 'num > left / 5' to avoid false reports. (CID 68117)

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

wpa_supplicant/interworking.c

@@ -508,20 +508,25 @@ static struct nai_realm * nai_realm_parse(struct wpabuf *anqp, u16 *count)
 	struct nai_realm *realm;
 	const u8 *pos, *end;
 	u16 i, num;
+	size_t left;
 
-	if (anqp == NULL || wpabuf_len(anqp) < 2)
+	if (anqp == NULL)
+		return NULL;
+	left = wpabuf_len(anqp);
+	if (left < 2)
 		return NULL;
 
 	pos = wpabuf_head_u8(anqp);
-	end = pos + wpabuf_len(anqp);
+	end = pos + left;
 	num = WPA_GET_LE16(pos);
 	wpa_printf(MSG_DEBUG, "NAI Realm Count: %u", num);
 	pos += 2;
+	left -= 2;
 
-	if (num * 5 > end - pos) {
+	if (num > left / 5) {
 		wpa_printf(MSG_DEBUG, "Invalid NAI Realm Count %u - not "
 			   "enough data (%u octets) for that many realms",
-			   num, (unsigned int) (end - pos));
+			   num, (unsigned int) left);
 		return NULL;
 	}