net: sntp: Check packet length in sntp_handler

Currently, the sntp_handler uses data in the UDP packet
regardless of the actual packet size. A OOB read can occur
if the packet is too small.
Fix it by checking the packet length before extracting
seconds from a SNTP packet.

Signed-off-by: Francois Berder <fberder@outlook.fr>
Reviewed-by: Jerome Forissier <jerome.forissier@arm.com>

net/sntp.c

@@ -64,6 +64,9 @@ static void sntp_handler(uchar *pkt, unsigned dest, struct in_addr sip,
 	if (dest != sntp_our_port)
 		return;
 
+	if (len < SNTP_PACKET_LEN)
+		return;
+
 	/*
 	 * As the RTC's used in U-Boot support second resolution only
 	 * we simply ignore the sub-second field.