pc: fix possible NULL pointer dereference in pc_machine_get_device_memory_region_size()

pc: fix possible NULL pointer dereference in pc_machine_get_device_memory_region_size()

QEMU will crash when device-memory-region-size property is read if ms->device_memory
wasn't initialized yet.

Crash can be reproduced with:
 $QEMU -preconfig -qmp unix:qmp_socket,server,nowait &
 ./scripts/qmp/qom-get -s qmp_socket /machine.device-memory-region-size

Instead of crashing return 0 if ms->device_memory hasn't been initialized.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1560174635-22602-1-git-send-email-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>