buildroot

iperf3: security bump to version 3.1.3

Fixes:
ESNET-SECADV-2016-0001 - A malicious process can connect to an iperf3
server and, by sending a malformed message on the control channel,
corrupt the server process's heap area.  This can lead to a crash (and a
denial of service), or theoretically a remote code execution as the user
running the iperf3 server. A malicious iperf3 server could potentially
mount a similar attack on an iperf3 client.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>