renderer: check detach_iov interface

check the return values aren't NULL

src/vrend_renderer.c

@@ -3307,7 +3307,9 @@ void vrend_renderer_resource_detach_iov(int res_handle,
    if (!res) {
       return;
    }
+   if (iov_p)
      *iov_p = res->iov;
+   if (num_iovs_p)
      *num_iovs_p = res->num_iovs;
 
    res->iov = NULL;