"read_size".
- Changed some int vars in map_ptr() needed to be int32 vars.
- All callers expect map_ptr() to return back the full "len" bytes,
  so we never shorten this value, even near EOF--the read will
  just get zero-padded, as needed.
- Die with an error if someone calls map_ptr() with a len < 0, or
  if read_size is computed as a value < 0.