vk/device: Provide proper NULL handling in anv_device_free

The Vulkan spec does not specify that the free function provided to
CreateInstance must handle NULL properly so we do it in the wrapper.  If
this ever changes in the spec, we can delete the extra 2 lines.

src/vulkan/device.c

@@ -1057,6 +1057,9 @@ void
 anv_device_free(struct anv_device *             device,
                 void *                          mem)
 {
+   if (mem == NULL)
+      return;
+
    return device->instance->pfnFree(device->instance->pAllocUserData,
                                     mem);
 }