问无法解决此c程序中的内存管理(valgrind)问题

EN

#define RIALLOCA(buf, newsize) buf = realloc(buf, newsize);

char *mystrcat(char *buf, size_t sz, char *first, ...) {
    va_list l;
    va_start(l, first);

    buf = malloc(strlen(buf) + 1);

    if (sz < strlen(first) + 1) {
        sz += (strlen(first) + 1);
    }

    RIALLOCA(buf, sz + 1 + 16);
    strncat(buf, first, strlen(first));

    char *nextString = va_arg(l, char *);

    while (nextString != NULL) { 
        // sz += strlen(nextString);
        RIALLOCA(buf, strlen(buf) + strlen(nextString) + 1 + 16);
        strncat(buf, nextString, strlen(nextString));
        nextString = va_arg(l, char *);
    }   

    va_end(l);

    return buf;
}   

int main(int argc, char *argv[]) {
    if (argc != 7) {
        printf("troppi pochi argomenti\n");
        return -1; 
    }   
    char *buffer = NULL;
    RIALLOCA(buffer, 16);  // macro che effettua l'allocazione
    buffer[0] = '\0';
    buffer = mystrcat(buffer, 16, argv[1], argv[2], argv[3], argv[4], argv[5],
                      argv[6], NULL);
    printf("%s\n", buffer);
    free(buffer);     
    return 0;
}