Why is the segmentation Fault error occuring

I am getting error Segmentation fault (core dumped)

i have narrowed it done to these lines in function threadx

    while (colatz_nums[j] != 1)
         {j++;
            if ((m % 2)==0)
            { colatz_nums[j]  = m/2;}

            else
            {colatz_nums[j]  = 3 * m +1;}
        } 

if I remove these lines I do not get the error. I added a test while loop and it worked So it must be something in these lines. Please point out the error

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>  // pid_t
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/mman.h>
#include <sys/fcntl.h>
#include <sys/stat.h>

#include <pthread.h>

#define N 2

void *thread (void *vargp);
void *threadx(void *vargp);

char **ptr;

int fib_nums[25] ; 
int colatz_nums[25];
int last_collatz = 0;

int main()
{

    int i;
    pthread_t tid[2];
    char *msgs[N] = {
    "Hello from foo",
    "Hello from bar"
    };

printf("Parent thread started with PID= %d and parent PID %d\n", getpid(), getppid());

    ptr = msgs;

    pthread_create(&tid[0], NULL, thread, (void *)1);
    printf(" 1st thread started with PID= %d and parent PID %d\n", getpid(), getppid());


    pthread_create(&tid[1], NULL, threadx, (void *)2 );
    printf("2nd  thread started with PID= %d and parent PID %d\n", getpid(),      getppid());

    pthread_join(tid[1], NULL);
    pthread_join(tid[0], NULL);
}

void *thread(void *vargp)
{
    int myid = (int)vargp;
    static int cnt = 0;
    printf(" thread ");

int i=cnt;
for (;i <10 ;i=i+1)
{
   printf("[%d]  %d\n",myid, i);
   sleep(cnt);
}

    return NULL;
}

void *threadx(void *vargp )
{ 
    int myid = (int)vargp;
    static int cnt = 0;
    printf(" threadx \n" );
int j = 0;
int m = 8;
colatz_nums[0] = 8;

while (colatz_nums[j] != 1)
        {j++;
            if ((m % 2)==0)
            { colatz_nums[j]  = m/2;}

            else
            {colatz_nums[j]  = 3 * m +1;}
        }
last_collatz = j;

 for (j=0; j <= last_collatz; j++)
    printf ( " j %d",colatz_nums[j]);



printf ( "\n");
 return NULL;

}