ovn-nbctl: fix sigsegv when nexthop arg is missing

running the following command will result core dump

$ ovn-nbctl lr-policy-add lr0 2000 ip4.src==192.168.0.15 reroute
Segmentation fault (core dumped)

This commit add the missing return when the nexthop is missing

with the fix running the same comand will result the currect output
$ ovn-nbctl lr-policy-add lr0 2000 ip4.src==192.168.0.15 reroute
ovn-nbctl: Nexthop is required when action is reroute.

Signed-off-by: Moshe Levi <moshele@nvidia.com>
Signed-off-by: Numan Siddique <numans@ovn.org>

(cherry-picked from master commit 5cdfece)

utilities/ovn-nbctl.c

@@ -3622,6 +3622,7 @@ nbctl_lr_policy_add(struct ctl_context *ctx)
     if (!strcmp(action, "reroute")) {
         if (ctx->argc < 6) {
             ctl_error(ctx, "Nexthop is required when action is reroute.");
+            return;
         }
     }