glGetString causes null exception
This code, with glad causes a null exception, as in `glad.c`, the function id defined as `NULL`. 

```c
#include <glad/glad.h>
#include <GLFW/glfw3.h>

int main(void) {
    GLFWwindow* window;

    /* Initialize the library */
    if (!glfwInit())
        return -1;
    
    /* Create a windowed mode window and its OpenGL context */
    window = glfwCreateWindow(640, 480, "Hello World", NULL, NULL);
    if (!window) {
        glfwTerminate();
        return -1;
    }

    /* Make the window's context current */
    glfwMakeContextCurrent(window);

    /* Loop until the user closes the window */
    while (!glfwWindowShouldClose(window)) {
        /* Render here */
        glClear(GL_COLOR_BUFFER_BIT);
        printf("%s\n", glGetString(GL_VERSION));

        /* Swap front and back buffers */
        glfwSwapBuffers(window);

        /* Poll for and process events */
        glfwPollEvents();
    }

    glfwTerminate();
    return 0;
}
```
Instead of glad, when I include the platform headers `<gl/gl.h>` it works fine. 

I am sure I am missing something, but am not sure as to what it is. Help would be appreciated, thanks!