target/s390x: Fix broken user mode

Returning NULL from get_max_cpu_model results in a SIGSEGV runtime error.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170130131517.8092-1-sw@weilnetz.de>
Cc: qemu-stable@nongnu.org
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>