gladLoadVulkan returns 0 with -O3 and GCC
I've just begun to learn Vulkan, and I'm encountering strange behavior. It may be my code or it may be Glad. With `-O2` the test case runs without giving an error. With `-O3` it exhibits the behavior mentioned above.

[vktest.zip](https://github.com/Dav1dde/glad/files/3824815/vktest.zip)