net: dns: Check DNS answer properly

The dns_unpack_answer() did not check the length of the message
properly which can cause out of bounds read.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

subsys/net/lib/dns/dns_pack.c

@@ -134,7 +134,7 @@ int dns_unpack_answer(struct dns_msg_t *dns_msg, int dname_ptr, uint32_t *ttl,
 	 *
 	 * See RFC-1035 4.1.3. Resource record format
 	 */
-	rem_size = dns_msg->msg_size - dname_len;
+	rem_size = dns_msg->msg_size - dns_msg->answer_offset - dname_len;
 	if (rem_size < 2 + 2 + 4 + 2) {
 		return -EINVAL;
 	}