gdkvulkancontext: remove incorrect vkDestroySwapchainKHR

Confusingly, the function that creates the next swapchain also "retires"
the current one. Trying to destroy it again results in closing an fd we
no longer own.

Fixes #6969

gdk/gdkvulkancontext.c

@@ -484,9 +484,8 @@ gdk_vulkan_context_check_swapchain (GdkVulkanContext  *context,
 
   if (priv->swapchain != VK_NULL_HANDLE)
     {
-      vkDestroySwapchainKHR (device,
-                             priv->swapchain,
-                             NULL);
+      /* priv->swapchain was just retired by vkCreateSwapchainKHR */
+      priv->swapchain = VK_NULL_HANDLE;
       for (i = 0; i < priv->n_images; i++)
         {
           cairo_region_destroy (priv->regions[i]);
@@ -523,7 +522,6 @@ gdk_vulkan_context_check_swapchain (GdkVulkanContext  *context,
     {
       g_set_error (error, GDK_VULKAN_ERROR, GDK_VULKAN_ERROR_NOT_AVAILABLE,
                    "Could not create swapchain for this surface: %s", gdk_vulkan_strerror (res));
-      priv->swapchain = VK_NULL_HANDLE;
       return FALSE;
     }