Glad generated loader does not correctly handle GL implementations with 0 extensions
`get_exts` treats an implementation having 0 extensions as an error case even though the spec makes no guarantees about the number of extensions provided by an implementation:
```c
    // generated glad.c - get_exts
    ...
        int index;
        num_exts_i = 0;
        glGetIntegerv(GL_NUM_EXTENSIONS, &num_exts_i);
        if (num_exts_i > 0) {
            exts_i = (char **)malloc((size_t)num_exts_i * (sizeof *exts_i));
        }

        if (exts_i == NULL) {
            return 0;
        }

        for(index = 0; index < num_exts_i; index++) {
            const char *gl_str_tmp = (const char*)glGetStringi(GL_EXTENSIONS, index);
            size_t len = strlen(gl_str_tmp);

            char *local_str = (char*)malloc((len+1) * sizeof(char));
            if(local_str != NULL) {
                memcpy(local_str, gl_str_tmp, (len+1) * sizeof(char));
            }
            exts_i[index] = local_str;
        }
    return 1;
```
Admittedly this is not that big of a deal because practically every "real" GL implementation has at least one extension, but I ran into it while testing a program using glad against the translation layer I'm working on (which currently advertises having no extensions).