my c program crashes on 16.04, but not on 14.04

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>
#define BUF_ROWS 100
#define BUF_COLS 64
/*@
 * this demo code can run on ubuntu 14.04 (32bit & 64bit) and mac osx (64bit)
 * But NOT on ubuntu 16.04 (64bit) as tested !
 */
#define dbg(...) fprintf(stderr,__VA_ARGS__)
void split_r(char* str, char* delim, char* toks[], size_t* sz)
{
    char* x, *sp;
    int i;
    x = strtok_r(str, delim, &sp);
    i = 0;
    while (x) {
        toks[i] = malloc(strlen(x)+1);
        strcpy(toks[i], x);
        i++;
        x = strtok_r(NULL, delim, &sp);
    }
    *sz = i;
}

int main(int argc, char** argv) {
    int gCnt, done =0;
    char ibuf[4096];

    char delim[] = " ";
    char* toks[8] = {NULL};
    size_t n;
    char** in;
    FILE* fp=fopen(argv[1],"r");
    if(!fp) { fprintf(stderr,"open %s file err. \n",argv[1]); exit(1); }

    while(!done) {
        gCnt=0;
        in = malloc(BUF_ROWS * BUF_COLS * sizeof(char*) );
        while (gCnt < BUF_ROWS*BUF_COLS) {
            if(fgets(ibuf, 4096, fp)) {
                ibuf[strcspn(ibuf, "\n")] = '\0';
                in[gCnt]=malloc(strlen(ibuf)+1);
                strcpy(in[gCnt], ibuf);
                char tm[128];
                strcpy(tm, in[gCnt]);
                dbg("In:'%s'\n", tm);
                split_r(tm, delim, toks, &n);
                if( n != 4) {
                 fprintf(stderr,"Fatal:in parse line '%s' to split n=%ld.\n", tm, n), exit(1);
                }
                gCnt++;
            } else {
                done=1;
                break;
            }

        }
        pid_t pid;
        if((pid=fork()) == 0) {
          // do sth
          exit(0);
        }
        wait(NULL);
        for (int i=0; i<gCnt; i++)
            printf("%s\n",in[i]), fflush(0);
        for(int i=0; i< BUF_ROWS * BUF_COLS; i++) free(in[i]);
        free(in);
 } // end of done
}

This program takes a input file in which each line has 4 tokens (strings). When I run it with a input file that contains 10000 lines, it always chokes at somewhere about 5000 ~ 6000th line in the input file on ubuntu 16.04. But it never chokes on ubuntu 14.04 & macbook pro -10.12.5. the input file contains the data something like that below ( each line has 4 strings only )
1 + 1 = 2
2 * 2 = 4
4 - 1 = 3
6 / 3 = 2
etc...

please let me know if any more information needed via my email
<email address hidden>

Thank you for attentions !!!

Best Regards

MJ

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-image-4.10.0-37-generic 4.10.0-37.41~16.04.1
ProcVersionSignature: Ubuntu 4.10.0-37.41~16.04.1-generic 4.10.17
Uname: Linux 4.10.0-37-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: amd64
CurrentDesktop: GNOME-Flashback:Unity
Date: Sat Oct 28 21:24:13 2017
InstallationDate: Installed on 2016-01-11 (656 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
SourcePackage: linux-hwe
UpgradeStatus: No upgrade log present (probably fresh install)