tests/vulkan: fix bug when VK_KHR_surface is missing

We call vkDestroySurfaceKHR indiscriminately, even if VK_KHR_surface was
not even enabled. (Since it's listed under `opt_extensions`, this is a
valid case)

Easiest way to work around this special case is to just avoid calling
vkDestroySurfaceKHR if we never created a surface, since in that case
`surf` will still be NULL.

src/tests/vulkan.c

@@ -287,7 +287,8 @@ int main()
         pl_log_level_update(log, PL_LOG_INFO);
     }
 
-    vkDestroySurfaceKHR(inst->instance, surf, NULL);
+    if (surf)
+        vkDestroySurfaceKHR(inst->instance, surf, NULL);
     pl_vk_inst_destroy(&inst);
     pl_log_destroy(&log);
     free(devices);