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: NIgor Mammedov <imammedo@redhat.com>
Message-Id: <20190624090200.5383-1-imammedo@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

hw/i386/pc.c

@@ -2553,7 +2553,11 @@ pc_machine_get_device_memory_region_size(Object *obj, Visitor *v,
                                          Error **errp)
 {
     MachineState *ms = MACHINE(obj);
-    int64_t value = memory_region_size(&ms->device_memory->mr);
+    int64_t value = 0;
+
+    if (ms->device_memory) {
+        value = memory_region_size(&ms->device_memory->mr);
+    }
 
     visit_type_int(v, name, &value, errp);
 }