为什么在评论 malloc() 调用时会出现分段错误（核心转储）？

int main()
{
    struct stuff
    {
        int num;
    }*foo;

    // If I comment the line below, I get core dump error.
    // Why?
    foo = (struct stuff *)malloc(sizeof(struct stuff));
    (*foo).num = 7;

    printf("%d\n", (*foo).num);
}