Fix Vulkan triangle test segfault on resize

The command buffer handle was not reset after being freed on window
resize, leading to a segfault when the stale handle was used.

README.md

@@ -199,6 +199,7 @@ skills.
  - Mário Freitas
  - GeO4d
  - Marcus Geelnard
+ - Charles Giessen
  - Stephen Gowen
  - Kovid Goyal
  - Eloi Marín Gratacós

tests/triangle-vulkan.c

@@ -2118,6 +2118,7 @@ static void demo_cleanup(struct demo *demo) {
 
     if (demo->setup_cmd) {
         vkFreeCommandBuffers(demo->device, demo->cmd_pool, 1, &demo->setup_cmd);
+        demo->setup_cmd = VK_NULL_HANDLE;
     }
     vkFreeCommandBuffers(demo->device, demo->cmd_pool, 1, &demo->draw_cmd);
     vkDestroyCommandPool(demo->device, demo->cmd_pool, NULL);