zink: account for kopper dt not having a swapchain when pruning batch usage

this could be pending deletion

cc: mesa-stable

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37379>

src/gallium/drivers/zink/zink_kopper.c

@@ -1240,6 +1240,10 @@ void
 zink_kopper_prune_batch_usage(struct kopper_displaytarget *cdt, const struct zink_batch_usage *u)
 {
    struct kopper_swapchain *cswap = cdt->swapchain;
+
+   if (!cswap)
+      return;
+
    swapchain_prune_batch_usage(cswap, u);
    for (cswap = cdt->old_swapchain; cswap; cswap = cswap->next)
       swapchain_prune_batch_usage(cswap, u);