On s390 we don't use vvar. Therefore vvar_size in not been initialized
and the value remains VVAR_BAD_SIZE.

Fix the BUG() statement to also cover this case.
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>