avoid invalid ovn-nbctl daemon socket path

[zhangzujian]

• Make sure you have followed Kube-OVN Code Style.

What type of this PR

• Bug fixes

In some scenarios, the environment variable OVN_NB_DAEMON contains output of command ovn-nbctl show:

E0811 14:09:33.118843 7 ovn-nbctl-legacy.go:1776] failed to access ovn-nb from daemon, "2022-08-11T06:09:33Z|00001|socket_util_unix|WARN|Unix socket name /var/run/ovn/ovn
-nbctl.17.ctl\nswitch ff84faad-84c6-4482-9848-d5418934deda (ovn-default)\n    port devops-api-cfbcdd76b-ndf2b.cpaas-system\n        addresses: [\"00:00:00:61:A5:AD 10.200.0.112
\"]\n    port devops-controller-757f46b86d-dzgbr.cpaas-system\n        addresses: [\"00:00:00:EB:4D:66 10.200.0.46\"]...: could not connect to ovn-nbctl daemon (File name too long); unset OVN_NB_DAEMON to avoid using daemon\n", "exit status 1"
W0811 14:09:33.119636 7 controller.go:73] ovn-nbctl daemon doesn't return, start a new daemon
...
E0811 16:14:24.525177       7 external_vpc.go:19] list lr failed2022-08-11T08:14:24Z|00001|socket_util_unix|WARN|Unix socket name /var/run/ovn/ovn-nbctl.30856.ctl
switch ff84faad-84c6-4482-9848-d5418934deda (ovn-default)
    port icarus-6457b7b867-wltzk.cpaas-system
        addresses: ["00:00:00:C4:CF:12 10.200.0.51"]
    port catalog-operator-7d97f8f9f8-t9zrl.cpaas-system
        addresses: ["00:00:00:68:F9:4A 10.200.0.134"]
    port devops-controller-757f46b86d-dzgbr.cpaas-system
...
    port razor-6f78bbf7dc-m4mq8.cpaas-system
        addresses: ["00:00:00:33:17:91 10.200.0. is longer than maximum 107 bytes

This patch adds a path check before setting the environment variable.