selinux: early exit in mac_selinux_maybe_reload if not initialized

Binaries might not initialize SELinux, e.g. when they normally do not
create files with the SELinux default context.
If they, via an internal libary function, call a _label() function,
mac_selinux_maybe_reload() gets called. Since the SELinux status page
has not been opened, selinux_status_updated() will fail with EINVAL.

This affects particularly test binaries.

Just exit early and avoid confusing debug logs.