Got this assertion:

    NetworkManager[12939]: <debug> [1536917977.4868] active-connection[0x563d8fd34540]: set state deactivated (was deactivating)
    ...
    NetworkManager[12939]: nm-openvpn[1106] <info>  openvpn[1132]: send SIGTERM
    NetworkManager[12939]: nm-openvpn[1106] <info>  wait for 1 openvpn processes to terminate...
    NetworkManager[12939]: nm-openvpn[1106] <warn>  openvpn[1132] exited with error code 1
    NetworkManager[12939]: <info>  [1536917977.5035] vpn-connection[0x563d8fd34540,2fdeaea3-975f-4325-8305-83ebca5eaa26,"my-openvpn-Red-Hat",0]: VPN plugin: requested secrets; state disconnected (9)
    NetworkManager[12939]: plugin_interactive_secrets_required: assertion 'priv->vpn_state == STATE_CONNECT || priv->vpn_state == STATE_NEED_AUTH' failed

Meaning. We should either ensure that secrets_required_cb() signal callback
is disconnected from proxy's signal, or we gracefully handle callbacks at
unexpected moments. Do the latter.

(cherry picked from commit 92344dd0)
(cherry picked from commit 011dd919)