object: add extra sanity checks

Type system checked that children class_size >= parent class_size, but
not instances. Fix that.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200110153039.1379601-2-marcandre.lureau@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qom/object.c

@@ -307,6 +307,7 @@ static void type_initialize(TypeImpl *ti)
         int i;
 
         g_assert(parent->class_size <= ti->class_size);
+        g_assert(parent->instance_size <= ti->instance_size);
         memcpy(ti->class, parent->class, parent->class_size);
         ti->class->interfaces = NULL;
         ti->class->properties = g_hash_table_new_full(