valgrind returns several invalid fread/fwrite/free()

leaky_recover.c

/**
 * recover.c
 *
 * Computer Science 50
 * Problem Set 4
 *
 * Recovers JPEGs from a "card.raw" forensic image.
 */

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

typedef uint8_t  BYTE;

void showmethepix(FILE* source, FILE* target, BYTE* buffer, int ctr, char* imagenr);

int main(void)
{
    // open forensic image file
    FILE* source = fopen("card.raw", "r");
    if (source == NULL)
    {
        printf("Could not open card.raw\n");
        return 2;
    }

    // create counter for filename, buffer to store filename
    int ctr = 0;
    char imagenr[8];
    
    // sprintf filename to buffer, open outfile and verify opening
    sprintf(imagenr,"%i.dmp", ctr);
    FILE* target = fopen(imagenr, "a");
    if (target == NULL)
    {
        fclose(source);
        fprintf(stderr, "Could not create %s.\n", imagenr);
        return 3;
    }
    
    // read target file into buffer
    BYTE *buffer = malloc (512 * sizeof(BYTE));
    if (buffer == NULL)
    {
        printf ("Could not generate buffer storage\n");
        fclose(source);
        fclose(target);
    }
    
    // check for JPG magic numbers, zeroes block
    showmethepix(source, target, buffer, ctr, imagenr);

    // wrap up upon return (i.e., when source file is only 0x00)
    free(buffer);
    fclose(source);
    fclose(target);
    remove("0.dmp");
    return 0;
}


void showmethepix(FILE* source, FILE* target, BYTE* buffer, int ctr, char* imagenr)
{
    fread(buffer, 512, 1, source);
    
    // if block contains info (i.e., not zeroes), write to target outfile
    if (!feof(source))
    {
        // initialize new file if block starts with .jpg magic numbers
        if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] >= 0xe0 && buffer[3] <= 0xef))
        {
            fclose(target);
            sprintf(imagenr,"%'.3i.jpg", ctr);
            target = fopen(imagenr, "a");
            ctr++;
        }
        
        // write chunk to outfile and match next chunk
        fwrite(buffer, 512, 1, target);
        showmethepix(source, target, buffer, ctr, imagenr);
    }
}