demos: Fix bug cube.c
demos: Fix bug cube.c

Fixed cube.c crashing on Windows when minimized.

Change-Id: Ia4a6578f7da6c1f7a10977b8343099df4c5c69fb

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools