test-sflow: Fix memory leak in main function.

test-sflow: Fix memory leak in main function.

Reported by valgrind on test case 886.

 912 (24 direct, 888 indirect) bytes in 1 blocks are definitely lost
    at malloc
    by xmalloc (util.c:112)
    by unixctl_server_create (unixctl.c:250)
    by test_sflow_main (test-sflow.c:688)
    by ovstest_wrapper_test_sflow_main__ (test-sflow.c:786)
    by ovs_cmdl_run_command (command-line.c:121)
    by main (ovstest.c:132)

 1,500 bytes in 1 blocks are definitely lost
    at malloc
    by xmalloc (util.c:112)
    by ofpbuf_init (ofpbuf.c:124)
    by test_sflow_main (test-sflow.c:696)
    by ovstest_wrapper_test_sflow_main__ (test-sflow.c:786)
    by ovs_cmdl_run_command (command-line.c:121)
    by main (ovstest.c:132)

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>