vmdk: Clean up bdrv_open_child() return value check

bdrv_open_child() may return NULL.
Usually return value is checked for this function.
Check for return value is more reliable.

Fixes: 24bc15d ("vmdk: Use BdrvChild instead of BDS for references to extents")

Signed-off-by: Dmitry Frolov <frolov@swemel.ru>
Message-ID: <20230831125926.796205-1-frolov@swemel.ru>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/vmdk.c

@@ -1207,7 +1207,7 @@ static int vmdk_parse_extents(const char *desc, BlockDriverState *bs,
                                       bs, &child_of_bds, extent_role, false,
                                       &local_err);
         g_free(extent_path);
-        if (local_err) {
+        if (!extent_file) {
             error_propagate(errp, local_err);
             ret = -EINVAL;
             goto out;