test-netflow: Fix memory leak reported by valgrind.

test-netflow: Fix memory leak reported by valgrind.

Test case 890: ofproto-dpif - NetFlow flow expiration - IPv4 collector
Valgrind reports two leaks below:
    unixctl_server_create (unixctl.c:250)
    test_netflow_main (test-netflow.c:200)
    ovstest_wrapper_test_netflow_main__ (test-netflow.c:301)
    ovs_cmdl_run_command (command-line.c:121)
    main (ovstest.c:132)
and
    ofpbuf_init (ofpbuf.c:124)
    test_netflow_main (test-netflow.c:208)
    ovstest_wrapper_test_netflow_main__ (test-netflow.c:301)
    ovs_cmdl_run_command (command-line.c:121)
    main (ovstest.c:132)

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Co-authored-by: Daniele Di Proietto <diproiettod@vmware.com
Signed-off-by: Ben Pfaff <blp@ovn.org>