vcl: fix iperf3 server crash issue when it runs over vpp host stack.

vcl: fix iperf3 server crash issue when it runs over vpp host stack.

Issue:

Let iperf3 server run via ldp and vcl on top of vpp's host stack. If
iperf3 client connects this iperf3 server with tcp MSS setting option,
iperf3 server will always crash.

Root cause:

When MSS option is specified by iperf3 client, iperf3 server will
recreate the listening socket firstly, then call setsockopt() to set MSS
immediately. Iperf3 code can be referred here:
https://github.com/esnet/iperf/blob/58332f8154e2140e40a6e0ea060a418138291718/src/iperf_tcp.c#L186.
However, in vcl layer vpp_evt_q of this recreated session is not
allocated yet. So iperf3 server crashes with vpp_evt_q null pointer access.

Fix:

Add session vpp_evt_q null pointer check in vcl_session_transport_attr().
Add a vcl test case for this MSS option scenario.

Type: fix
Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Change-Id: I2863bd0cffbe6e60108ab333f97c00530c006ba7