bootdevice: fix segment fault when booting guest with '-kernel' and '-initrd'

bootdevice: fix segment fault when booting guest with '-kernel' and '-initrd'

Reproducer:

 $./qemu-system-x86_64 --enable-kvm -kernel /home/vmlinuz-2.6.32.12-0.7-default \
  -initrd /home/initrd-2.6.32.12-0.7-default -append \
 "root=/dev/ram rw console=ttyS0,115200" -dtb guest.dtb -vnc :10 --monitor stdio -smp 2
QEMU 2.2.50 monitor - type 'help' for more information
(qemu) Segmentation fault (core dumped)

Reported-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Message-Id: <1425001784-6752-1-git-send-email-arei.gonglei@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>