Finding one Hamiltonian cycle in graph error - c

Question

I have a program in C which reads definition of graph from file, search for Hamiltonian cycle (only one) and prints it on screen if found. Problem is that program is crashing when I'm trying to find cycle in graphs with 30 and more vertices (for 30 vertices it sometimes shows cycle end get crashed (with different saturations), for more get crashed instantly). When I try to debug it stops at free() function and shows SIGTRAP signal. What can I do to fix that? Here's my code:
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <time.h>
typedef struct Lista
{
struct Lista * next;
int v;
} Lista;
Lista * stos;
bool *visited;
int wierzcholki, krawedzie;
int *S;
int sptr;
Lista **graf;
bool czyZnaleziono = false;
int DFSHamilton(int v)
{
if (!czyZnaleziono)
{
int i;
bool test;
Lista *p;
S[sptr++] = v;
if(sptr < wierzcholki)
{
visited[v] = true;
for(p = graf[v]; p; p = p->next)
if(!visited[p->v]) DFSHamilton(p->v);
visited[v] = false;
}
else
{
test = false;
for(p = graf[v]; p; p = p->next)
if(!(p->v))
{
test = true;
break;
}
if(test)
{
printf("Hamiltonian Cycle : ");
for(i = 0; i < sptr; i++)
{
printf("%d ",S[i]);
}
printf("0\n");
czyZnaleziono = true;
}
}
sptr--;
}
}
int main()
{
FILE *plik;
Lista *p, *r;
plik = fopen("rzeczy40-95.txt", "r");
double start, stop, czas;
int i, j, w1, w2;
sptr = 0;
visited = malloc(krawedzie*sizeof(bool));
if (plik == NULL)
{
printf("Nie mozna odnalezc pliku");
return 0;
}
else
{
fscanf(plik, "%d %d", &wierzcholki, &krawedzie);
S = malloc(wierzcholki*sizeof(int));
graf = malloc(wierzcholki*sizeof(Lista));
for(i=0; i<wierzcholki; i++)
{
graf[i] = NULL;
visited[i] = false;
}
for(j=0; j<krawedzie; j++)
{
fscanf(plik,"%d %d", &w1, &w2);
p = malloc(sizeof(Lista));
r = malloc(sizeof(Lista));
p->v = w2;
p->next = graf[w1];
graf[w1] = p;
r->v = w1;
r->next = graf[w2];
graf[w2] = r;
}
fclose(plik);
DFSHamilton(0);
free(visited);
free(S);
for (i = 0; i < wierzcholki; i++)
{
free(graf[i]);
}
free(graf);
return 0;
}
}