链表合并，编译可以，无法输出是怎么回事？

#include<stdio.h>
#include <string.h>
#include <stdlib.h>

typedef struct Stu
{
    int id;
    int results;
    struct Stu *next;
}Stu;

Stu *create_Stu(int n)
{
    Stu *head=(Stu*)malloc(sizeof(Stu));
    if(head==NULL)
        return NULL;
    head->id=-1;
    head->results=-1;
    head->next=NULL;
    Stu *temp=head;
    int i;
    for(i=0;i<n;i++)
    {
        Stu *new_node=(Stu*)malloc(sizeof(Stu));
        if(new_node==NULL)
            return 0;
        scanf("%d",&new_node->id);
        scanf("%d",&new_node->results);
        new_node->next=NULL;
        temp->next=new_node;
        temp=new_node;
    }
    return head;
}

Stu *merge_Stu(Stu *s1,Stu *s2)
{
    if(s1==0||s2==0)
        return 0;
    Stu *s1_tail=s1;   
    while(s1_tail->next!=0)
        s1_tail=s1_tail->next;
    s1_tail->next=s2->next;
    free(s2);
    return s1;   
}

void sort_Stu(Stu *stu)
{
    if(stu==0)
        return;
    Stu *t1,*t2,*temp,t;
    t1=t2=temp=0;
    for(t1=stu->next;t1->next!=0;t1=t1->next)
    {
        temp=t1;
        for(t2=t1->next;t2!=0;t2=t2->next)
        {
            if(temp->id>t2->id)
                temp=t2;
        }
        if(temp!=t1)
        {
            t=*t1;
            *t1=*temp;
            *temp=t;
            t.next=t1->next;
            t1->next=t2->next;
            t2->next=t.next;
        }
    }   
}

void print_Stu(Stu *s)
{
    if(s==0||s->next!=0)
        return;
    Stu *node=s->next;
    while(node!=0)
    {
        printf("%d %d\n",node->id,node->results);
        node=node->next;
    }
}
void destory_Stu(Stu *s)
{
    if(s==0)
        return;
    Stu *p=s;
    Stu *p1=0;
    while(p!=0)
    {
        p1=p->next;
        free(p);
        p=p1;
    }
}


int main ()
{
    int N, M;
    scanf("%d%d", &N, &M);

 
    //创建两个链表
    Stu *students1 = create_Stu(N);
    Stu *students2 = create_Stu(M);

 
    //合并两个链表
    Stu *students = merge_Stu(students1, students2);

 
    //对新链表中的内容按学号升序排列并打印
    sort_Stu(students);
    print_Stu(students);

 
    destory_Stu(students);
    return 0;

 }