connman: fix a potential crash

cm_service_add() creates a CMService struct with a NULL
service_proxy field and then uses the asynchronous
g_dbus_proxy_new_for_bus() to create the proxy. So the
service_proxy may be NULL when cm_service_free() is called
before the proxy is actually created.

https://bugzilla.gnome.org/show_bug.cgi?id=770318