Fix freeing of vulkan feature structs is test context.

If the CreateVkBackendContext fails, we need to make the
VkphysicalDeviceFeatures2 struct at least has null filled into its
pNext so the FreeVulkanFeaturesStructs calls doesn't try to free
random memory.

Bug: skia:
Change-Id: I171d125ccba087cdd39595aca815d95463115b19
Reviewed-on: https://skia-review.googlesource.com/150139
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>

tools/gpu/vk/VkTestContext.cpp

@@ -177,6 +177,7 @@ public:
             };
             extensions = new GrVkExtensions();
             features = new VkPhysicalDeviceFeatures2;
+            memset(features, 0, sizeof(VkPhysicalDeviceFeatures2));
             if (!sk_gpu_test::CreateVkBackendContext(getProc, &backendContext, extensions,
                                                      features, &debugCallback)) {
                 sk_gpu_test::FreeVulkanFeaturesStructs(features);