header.c

struct malloc_chunk {
  struct malloc_chunk *next; // next pointer on free list
  size_t size;               // the size of this chunk
  unsigned free;             // > 0 if this pointer is free - useful for runtime assertions
}

struct malloc_chunk *
header(void *ptr)
{
  return (struct malloc_chunk *)ptr - sizeof(malloc_chunk);
}

left.c

struct malloc_chunk *left = (struct malloc_chunk *)(ptr - footer_left(ptr) - sizeof(size_t));

right.c

struct malloc_chunk *right = (struct malloc_chunk *)(ptr + header(ptr)->size);

I believe the address calculation in line 10 of header.c is incorrect. Your current calculation gets you ptr - sizeof(struct malloc_chunk) * sizeof(struct malloc_chunk) bytes. Either subtract one from ptr or use (char*)ptr instead.