hdf5_msb

Change the blah_blah_blah_md_header `index_length` member from
`uint64_t` to `size_t` because it describes the size of an in-core
structure as well as an on-disk one, and `size_t` is wide enough
to store the size of any in-core structure, while `uint64_t` may
be much too wide.  Check that `index_length` is no more than SIZE_MAX
after we read it.