Bad Ptr Yaklama?

#include <stdarg.h>
#include <conio.h>
#include <stdio.h>
#include <string.h>

char *MultiStrCat(char * first, ...);
int main(int argc, char* argv[])
{
    char *str1 ="Hello World1";
    char *str2 ="Hello World2";
    char *str3 ="Hello World3";
    char *StrMix;
    StrMix =MultiStrCat(str1,str2,str3);
    printf("%s\n",StrMix);
    getch();
    return 0;
}

char *MultiStrCat(char * first, ...)
{
    int totlen;
    char * item;
    int count = 0, sum = 0;
    item = first;
    va_list marker;
    va_start( marker, first );
    while(true)
    {
        count++;
        sum = sum+ strlen(item);
        item = (char*) va_arg( marker, char*);
        if (item ==NULL) {break;} //Hata burada
    }

    va_end(marker);
    char *buffer =new char[sum+1]; 
    *(buffer+0)='\0';
    strcpy(buffer,first);
    va_start( marker, first );
    for (int c=0;c<count-1;c++){
        item = va_arg( marker, char*);
        strcat(buffer,item);
    }
    return(buffer);
}