Doubly linked list implementation segfaulting

[jagged jacinth]

Hey all, I'm currently trying to implement doubly linked lists to use in my game project but I seem to have messed up some code logic as one of my assertions are failing. The function with it is this one:

typedef struct ListNode {
    void* data;
    struct ListNode* next;
    struct ListNode* prev;
} ListNode;
typedef struct List {
    int size;
    ListNode* head;
    ListNode* tail;
    size_t type;
} List;

void list_remove_nth(List* list, int n) {
    assert(n >= 0 && n < list->size);
    ListNode* cur = list->head;
    for (int i = 0; i < n && cur != NULL; i++) 
        cur = cur->next;
    if (cur == list->head) {
        list->head = cur->next;
        if (list->head != NULL)
            list->head->prev = NULL;
        else
            list->tail = NULL;
    } else if (cur == list->tail) {
        list->tail = cur->prev;
        list->tail->next = NULL;
    } else {
        cur->prev->next = cur->next;
        cur->next->prev = cur->prev;
    }
    free(cur);
    list->size--;
}

and I'm using this code to test it:

    List* li = list_init();
    int x = 4;
    int y = 8;
    list_push(li, &x);
    list_push(li, &x);
    list_push(li, &y);
    list_push(li, &x);

    printf("List size is %d\n", li->size);

    ListNode* cur = li->head;
    printf("Getting list elements with while loop\n");
    while (cur != NULL) {
        printf("%d\n", *(int*)cur->data);
        cur = cur->next;
    }

    printf("Getting list elements with for loop\n");
    for (int i = 0; i < li->size - 1; i++) {
        printf("%d: %d\n", i, *(int*)list_get_nth(li, i));
    }

    list_remove_nth(li, 0);
    list_remove_nth(li, 3);
    printf("List size after removing elements: %d\n", li->size);

    printf("New List elements (using for loop):\n");
    for (int i = 0; i < li->size - 1; i++) {
        printf("%d\n", *(int*)list_get_nth(li, i));
    }

    list_cleanup(li);

The full code is in this Pastebin, if anyone would like to see it: https://pastebin.com/NRa8uVz5

[opaque haven]

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[spare dust]

only here:

ListNode* cur = list->head;
for (int i = 0; i < n && cur != NULL; i++) 
    cur = cur->next;
if (cur == list->head) {    // (1)
    list->head = cur->next;

you have no proof that entering (1) implies cur != NULL (if the list->head is NULL from the start)

(not saying it is the segfault you're observing ; but I'd put more assert before accessing mutable things, and likely rephrase parts of the algo)