Make sure that Vulkan debug callback ptr is non-null.

src/gpu/vk/GrVkGpu.h

Index: src/gpu/vk/GrVkGpu.h
diff --git a/src/gpu/vk/GrVkGpu.h b/src/gpu/vk/GrVkGpu.h
index 8b8883bd047d44977bf6d12dae31b62a14a9a712..506b2379f6fecf5c2fe271c61d8ebbc4f4dbb89a 100644
--- a/src/gpu/vk/GrVkGpu.h
+++ b/src/gpu/vk/GrVkGpu.h
@@ -208,9 +208,6 @@ private:
 
     // These Vulkan objects are provided by the client, and also stored in fBackendContext.
     // They're copied here for convenient access.
-#ifdef ENABLE_VK_LAYERS
-    VkInstance                             fVkInstance;
-#endif
     VkDevice                               fDevice;
     VkQueue                                fQueue;    // Must be Graphics queue