Segmentation fault in hash table when inserting in linked list - c

Question

I want to store passwords imput by user in a hash table by how strong the password is. After reading first password, it has to be inserted in the hash table and than go back for reading password. Segmentation fault occurs after first iteration.
The insertion function is basically an insertion in a linked list, what could be wrong there ?
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
struct Node{
char* str;
struct Node* next;
};
int procent(char* str){
char aux;
for(int i = 0; i < strlen(str)-1; i++){
for(int j = i+1; j < strlen(str); j++){
if(str[i] > str[j]){
aux = str[i];
str[i] = str[j];
str[j] = aux;
}
}
}
int count = 1, chr = str[0];
for(int i = 0; i < strlen(str); i++){
if(str[i] != chr){
count++;
chr = str[i];
}
}
return count;
}
int hash_function(char* str){
int len = strlen(str), index;
if(len < 5){
index = 0;
}
else if(index >= 5 && index < 8){
index = 1;
}
else if(index >= 8 && index < 12){
index = 2;
}
else{
index = 3;
}
float rap = procent(str)/strlen(str);
if(rap < 0.5 && index > 0){
index--;
}
else if(rap > 0.5 && index < 3){
index++;
}
return index;
}
void insert(struct Node** head, char* str){
printf("Error");
struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));
strcpy(new_node->str, str);
new_node->next = NULL;
if(*head == NULL){
*head = new_node;
return;
}
else{
struct Node* last_node = *head;
while(last_node->next != NULL){
last_node = last_node->next;
}
last_node->next = new_node;
}
}
int main(){
struct Node *H[4];
for(int i = 0; i < 4; i++){
H[i] = NULL;
}
int raspuns, rap;
char pass[20];
while(1){
printf("Mai citesti ? 1-da/0-nu: ");
scanf("%d",&raspuns);
if(raspuns == 1){
printf("Parola: ");
scanf("%s", pass);
rap = procent(pass)/strlen(pass);
insert(&H[rap], pass);
}
else{
break;
}
}
for(int i = 0; i < 4; i++){
switch(i){
case 0:{printf("Foarte slabe: "); break;}
case 1:{printf("Slabe: "); break;}
case 2:{printf("Puternice: "); break;}
case 3:{printf("Foarte puternice: "); break;}
default: break;
}
while(H[i] != NULL){
printf("%s", H[i]->str);
H[i] = H[i]->next;
}
printf("\n");
}
return 0;
}