链表合并问题，没有报错，但运行时有些数据可以有些不可以，找不到哪里出错了

pb1 = pb1->next;
pb2->next = pa1;

#define _CRT_SECURE_NO_WARNINGS

#include<stdio.h>
#include<malloc.h>
#define LEN sizeof(struct student)

typedef struct student
{
    int num;
    int score;
    struct student *next;
}stu;

void print(stu *head);
void insert(stu *head, stu *p);
stu *creat();
void clear(stu *head);
stu *combine(stu *ahead, stu *bhead);

void main()
{
    stu *ahead, *bhead;
    printf("input list a:\n");
    ahead = creat();          //建立链表a
    printf("List a:\n");
    print(ahead);
    printf("\n\ninput list b:\n");
    bhead = creat();
    printf("List b:\n");    //建立链表b
    print(bhead);
    ahead = combine(ahead, bhead);  //合并链表
    printf("\n\nList:\n");
    print(ahead);
    clear(ahead);     //清除
}

stu *creat()
{
    stu *p, *head;
    head = (stu *)malloc(LEN);    //建立头结点
    head->next = NULL;
    printf("input number,score:\n");
    p = (stu *)malloc(LEN);    //建立数据节点
    scanf("%d %d", &p->num, &p->score);  //输入数据
    p->next = NULL;
    while (p->num != 0)
    {
        insert(head, p);    //按顺序输入
        p = (stu *)malloc(LEN);
        scanf("%d %d", &p->num, &p->score);
    }
    free(p);
    return(head);
}

void insert(stu *head, stu *p)
{
    stu *p0, *p1, *p2;
    p2 = head;
    p1 = head->next;
    p0 = p;
    while (p1 != NULL)
    {
        if (p0->num > p1->num)   //让p0在p1和p2之间
        {
            p2 = p1;
            p1 = p1->next;     //p1后移
        }
        else break;
    }
    p0->next = p1;     //p0,p1链接
    p2->next = p0;     //p2,p0链接
}

stu *combine(stu *ahead, stu *bhead)
{
    stu *pa1, *pa2, *pb1, *pb2;
    pa2 = ahead;
    pa1 = ahead->next;
    pb2 = pb1 = bhead->next;   //头结点用不到
    while (pb1&&pa1->next != NULL)
    {
        while ((pb1->num > pa1->num) && (pa1->next != NULL))
        {
            pa2 = pa1;        //往后移
            pa1 = pa1->next;  //后移
        }
        if (pb1->num <= pa1->num)
        {
            pa2->next = pb1;
            pb1 = pb1->next;
            pb2->next = pa1;     //节点连接
                //第二个链表后移
            pa2 = pb2;          //pa1移动
            pb2 = pb1;          //pb2后移
        }
    }
    if (pb1)       //非空已指向链表b尾
        pa1->next = pb1;    //b链表剩下的接到a链表后边
    free(bhead);
    return(ahead);
}

void print(stu *head)
{
    stu *p;
    p = head->next;
    while (p != NULL)
    {
        printf("%d %d\n", p->num, p->score);
        p = p->next;
    }
}

void clear(stu *head)
{
    stu *p;
    p = head->next;
    while (p != NULL)
    {
        head->next = p->next;
        free(p);
        p = head->next;
    }
    free(head);
}