threading: avoid invalid GIPA call in vkCreateDevice

Save VkInstance in layer_data so that we query vkCreateDevice with a valid
instance.
diff --git a/layers/threading.h b/layers/threading.h
index 8e90f0a..c3db346 100644
--- a/layers/threading.h
+++ b/layers/threading.h
@@ -189,6 +189,8 @@
 };
 
 struct layer_data {
+    VkInstance instance;
+
     debug_report_data *report_data;
     std::vector<VkDebugReportCallbackEXT> logging_callback;
     VkLayerDispatchTable *device_dispatch_table;