问C- double free

EN

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

struct strqueue {
    struct lnode *front;
    struct lnode *back;
    int length;
};

struct lnode {
    char *item;
    struct lnode *next;
};

StrQueue create_StrQueue(void) {
    struct strqueue *sq = malloc(sizeof(struct strqueue));
    sq->length = 0;
    sq->front = NULL;
    sq->back = NULL;
    return sq;
}

void destroy_nodes(struct lnode *l) {
    while (l!=NULL) {
        struct lnode *c = l;
        l=l->next;
        free(c);
    }
}

void destroy_StrQueue(StrQueue sq) {
    destroy_nodes(sq->front);
    free(sq);
}

void sq_add_back(StrQueue sq, const char *str) {
    struct lnode *n = malloc(sizeof(struct lnode));
    n->item = malloc(sizeof(char)*(strlen(str)+1));
    strcpy(n->item, str);
    n->next = NULL;
    if (sq->length == 0) {
        sq->front = n;
        sq->back = n;
    } else {
        sq->back->next = n;
        sq->back = n;
    }
    sq->length++;
}

char *sq_remove_front(StrQueue sq) {
   if (sq->front == NULL) {
      return NULL;
   } else {
      struct lnode *f = sq->front;
      char *temp = sq->front->item;
      sq->front = sq->front->next;
      sq->length--;
      //Delete the line below will not cause an error of not free all memory
      free(f->item);
      free(f);
      return temp;
   }
}

int sq_length(StrQueue sq) {
    return sq->length;
}