Odd segfault occuring on EOF exit of fgetc while loop

So I've combed through this thing pretty thoroughly using printf statements (on mobile) and have narrowed the location of the error down to the nested if (ret == EOF) statement in the read_file function. The data is successfully copied but segfaults on exit of the function. Any chance someone might be able to take a look for me?

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

#define MAX_BYTES 1024

typedef int8_t byte;

typedef struct file_buffer {
    
    char * file_name;
    void * raw_data;
    
    uint64_t file_size;
    uint64_t max_size;
    uint64_t temps_copied;
    uint16_t bytes_in_temp;
    uint8_t filled;
    
    FILE * fp;
    
} FB;



void * open_file(const char * file_name);

static FB * new_buffer(const char * file_name);

static void read_file(FB * fb, FILE * fp);

static void copy_buffer(FB * fb, byte * temp, const int size);

static void check_size(FB * fb, const int size);



int main()
{
    open_file("./src/MOCK_DATA.csv");
    return 0;
}


void * open_file(const char * file_name)
{
    FILE * fp;
    fp = fopen(file_name, "rb");
    //
    
    FB * buffer = new_buffer(file_name);
    
    read_file(buffer, fp);
    
    return buffer->raw_data;
}




static FB * new_buffer(const char * file_name)
{
    FB * fb = calloc(1, sizeof(FB));
    //
    
    fb->raw_data = calloc(MAX_BYTES, sizeof(byte));
    //
    
    size_t name_size = strlen(file_name) + 1;
    fb->file_name = malloc(sizeof(char) * name_size);
    //
    memcpy(fb->file_name, file_name,  name_size);
    
    return fb;
}



static void read_file(FB * fb, FILE * fp)
{
    byte temp[MAX_BYTES];
    byte nb = 0;
   
    int i = 0;
    int j = 0;
    int ret;
 
    while (ret = (nb = (fgetc(fp))))
    {
     if ((ret == EOF) || (j == (MAX_BYTES - 1)))
        {
            if (ret == EOF) {
                copy_buffer(fb, &temp[0], i);                
                return;
        }
        
        else {
            copy_buffer(fb, &temp[0], i);
            
            fb->bytes_in_temp = 0;
            j = 0;
            i = i + 1;
        }
        }
        fb->bytes_in_temp);
        
        temp[j] = nb;
        
        fb->bytes_in_temp++;
        j = j + 1;
    }
}



static void copy_buffer(FB * fb, byte * temp, const int size)
{
    check_size(fb, size);
    
    int start = MAX_BYTES * size;
    
    memcpy(&fb->raw_data[start], temp, fb->bytes_in_temp);
}



static void check_size(FB * fb, const int size)
{
    if (fb->temps_copied < size)
    {
        void * temp = realloc(fb->raw_data, (MAX_BYTES + fb->max_size) * sizeof(byte));
        
        fb->raw_data = temp;        
        fb->temps_copied = size;
        fb->max_size += MAX_BYTES;
    }
}