why does it have Segmentation fault: 11
int make_post(const User *author, User *target, char *contents) {
    if (author == NULL || target == NULL) {
        return 2;
    }
    for (int i = 0; i < MAX_FRIENDS; i++) {
        if (author->friends[i] == target) {
            Post *post = malloc(sizeof(Post));
            strcpy(post->author, author->name);
            strcpy(post->contents, contents);
            post->date = time(NULL);
            post->next = target->first_post;
            target->first_post = post;
            return 0;
        }
    }
    return 1;
}