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

src/gpu/vk/GrVkGpu.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrVkGpu_DEFINED
 #define GrVkGpu_DEFINED
 
@@ skipping 190 matching lines @@
     bool uploadTexDataOptimal(GrVkTexture* tex,
                               int left, int top, int width, int height,
                               GrPixelConfig dataConfig,
                               const SkTArray<GrMipLevel>&);
 
     SkAutoTUnref<const GrVkBackendContext> fBackendContext;
     SkAutoTUnref<GrVkCaps>                 fVkCaps;
 
     // 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
 
     // Created by GrVkGpu
     GrVkResourceProvider                   fResourceProvider;
     VkCommandPool                          fCmdPool;
     GrVkCommandBuffer*                     fCurrentCmdBuffer;
     VkPhysicalDeviceMemoryProperties       fPhysDevMemProps;
 
 #ifdef ENABLE_VK_LAYERS
     // For reporting validation layer errors
     VkDebugReportCallbackEXT               fCallback;
 #endif
 
     // Shaderc compiler used for compiling glsl in spirv. We only want to create the compiler once
     // since there is significant overhead to the first compile of any compiler.
     shaderc_compiler_t fCompiler;
 
 
     typedef GrGpu INHERITED;
 };
 
 #endif