libxlu: handle NULL pointer in xlu_cfg_destroy

libxlu: handle NULL pointer in xlu_cfg_destroy

Handle NULL pointer passed to xlu_cfg_destroy. libvirt calls it in its
libxlDriverConfigDispose function. If the XLU_Config was not initialized
yet for some reason a crash would occur on cleanup.
Avoid the crash just like libxl_ctx_free and xtl_logger_destroy do when
called from the same context.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>