int update_pic(User *user, const char *filename) {
    /* Check if filename is too long. */
    if (strlen(filename) > sizeof(user->profile_pic) - 1) {
        return 2;
    } elif (fopen(filename, "r") == NULL) {
        return 1;
    } else {
        user->profile_pic = filename;
        return 0;
    }
}
Change the filename for the profile pic of the given user. Return:
- 0 on success.
- 1 if the file does not exist or cannot be opened.
- 2 if the filename is too long.