ovs-ofctl: Fix coredump when using "add-groups" command.

When using ovs-ofctl add-groups with only "switch" argument, a coredump
is generated.  The main reason is that the command "ovs-ofctl add-groups"
need two arguments but the limitation of min-args of this command is
set to 1.

Fixes: 7395c05 ("Implement OpenFlow 1.1+ "groups" protocol.")
Submitted-at: #360
Signed-off-by: Wang Yibo <bobxxwang@126.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

utilities/ovs-ofctl.c

@@ -5041,7 +5041,7 @@ static const struct ovs_cmdl_command all_commands[] = {
     { "add-group", "switch group",
       1, 2, ofctl_add_group, OVS_RW },
     { "add-groups", "switch file",
-      1, 2, ofctl_add_groups, OVS_RW },
+      2, 2, ofctl_add_groups, OVS_RW },
     { "mod-group", "switch group",
       1, 2, ofctl_mod_group, OVS_RW },
     { "del-groups", "switch [group]",