链表归并排序出现溢出错误

所以我终于能够让测试函数工作，但我没有通过这个链表合并排序的测试函数。经过几个小时的调试，现在它变得最糟糕，出现以下溢出错误:

ConsoleApplication2.exe中0x01041719未处理异常:0xC00000FD: Stack overflow(参数:0x00000001, 0x006E2FC0).

#include <stdio.h>
#include <stdlib.h>
struct listnode {struct listnode * next; int key; };
struct listnode * merge(struct listnode * left, struct listnode * right)
{
    struct listnode * right2;
    if (left == NULL)
        return right;
    if (right == NULL)
        return left;
    if (left->key < right->key)
    {
        right2 = left;
        right2->next = merge(left->next, right);
    }
    else
    {
        right2 = right;
        right2->next = merge(left, right->next);
    }
    return right2;
}
struct listnode *sort(struct listnode * a)
{
    struct listnode * left, * right;
    if (a== NULL || a->next == NULL)
        return a;
    left = a; right = a->next;
    while (right!= NULL && right->next != NULL)
    {
        left = left->next;
        right = right->next->next;
    }
    right = left->next;
    left->next = NULL;
    return merge(sort(a), sort(right));
}

int main()
{
    long i;
    struct listnode *node, *tmpnode, *space;
    space = (struct listnode *) malloc(500000 * sizeof(struct listnode));
    for (i = 0; i < 500000; i++)
    {
        (space + i)->key = 2 * ((17 * i) % 500000);
        (space + i)->next = space + (i + 1);
    }
    (space + 499999)->next = NULL;
    node = space;
    printf("n prepared list, now starting sortn");
    node = sort(node);
    printf("n checking sorted listn");
    for (i = 0; i < 500000; i++)
    {
        if (node == NULL)
        {
            printf("List ended earlyn");
        }
        if (node->key != 2 * i)
        {
            printf("Node contains wrong valuen");
        }
        node = node->next;
    }
    printf("Sort successfuln");
    return 0;
}