Invalid pointer own get_next_line implementation

Question

In the last reading of the file, when it still has to return a line, and the line variable has the content of that line, an invalid pointer error occurs when doing free in the main but that line has content. On the other hand, if the free is placed outside the loop, it reads well, but I need it to be inside to avoid leaks.

int    main(void)
{
    int fd;
    char *line;

    fd = open("file.txt", O_RDONLY);
    while ((line = get_next_line(fd)) != NULL)
    {
        printf("%s", line);
        free(line);
    }
    printf("%s", line);
    free(line);
    return(0);
}

#include "get_next_line.h"

char    *cut_line(char  *line, char *new_line)
{
    int i;

    i = 0;
    while (line[i] != '\n' && line[i] != '\0')
        i++;
    if (line[i] == '\n')
        i++;
    while (line[i])
    {
        *new_line = line[i];
        line[i] = '\0';
        new_line++;
        i++;
    }
    while (*new_line)
    {
        *new_line = '\0';
        new_line++;
    }
    return (line);
}

char    *finalize_line(char **line, char **new_line)
{
    *line = cut_line(*line, *new_line);
    if (**new_line == '\0')
    {
        free(*new_line);
        *new_line = NULL;
    }
    if (**line == '\0' && !*new_line)
    {
        free(*line);
        *line = NULL;
        return (NULL);
    }
    return (*line);
}

char    *get_next_line(int fd)
{
    static char *new_line;
    char        *buffer;
    char        *line;
    int         read_bytes;

    buffer = ft_calloc(BUFFER_SIZE + 1, sizeof(char));
    if (!new_line)
        new_line = ft_calloc(BUFFER_SIZE + 1, sizeof(char));
    if (fd < 0 || BUFFER_SIZE <= 0 || !buffer || !new_line)
        return (free_null(&buffer, &new_line));
    line = ft_strjoin(new_line, buffer, 0);
    read_bytes = 1;
    while (!(ft_strchr(buffer, '\n')) && read_bytes > 0)
    {
        read_bytes = read(fd, buffer, BUFFER_SIZE);
        if (read_bytes == 0)
            break ;
        if (read_bytes == -1)
            return (free_null(&buffer, &new_line));
        buffer[read_bytes] = '\0';
        line = ft_strjoin(line, buffer, 1);
    }
    free(buffer);
    return (finalize_line(&line, &new_line));
}

char    *ft_strjoin(char *s1, char *s2, int flag)
{
    char    *aux_s1;
    int     i;
    int     j;

    if (!s1)
        s1 = ft_calloc(1, sizeof(char));
    if (!s2)
        s2 = ft_calloc(1, sizeof(char));
    aux_s1 = (char *)malloc(ft_strlen(s1) + ft_strlen(s2) + 1);
    if (!aux_s1)
        return (0);
    i = 0;
    while (s1[i] != '\0')
    {
        aux_s1[i] = s1[i];
        i++;
    }
    j = 0;
    while (s2[j] != '\0')
        aux_s1[i++] = s2[j++];
    aux_s1[i] = '\0';
    if (flag == 1)
        free(s1);
    return (aux_s1);
}

I have tried changing the function instead of traversing the pointer to traverse with an iterator but it also gives me an error. Also this happens with a buffer size of 10 or 100, if I use a buffer size of 1 or 10000 it works. I don't know if it's a problem with how I have the main structured or if it's something in the program itself.