hdf5_ep

[svn-r9101] Purpose:
    Bug fix

Description:
    1 - Dataset contiguous storage cache information had a bug where it was
possible to try to access invalid cache information if the cache wasn't filled
the first time it attempted to loop through the list of offset/length vectors.

    2 - Additionally, the contiguous storage cache information was being used
in certain circumstances from the chunked dataset I/O code path, which was
generally fatal since the chunk storage and contiguous storage information
were stored together in a union.


Solution:
    1 - Avoid special case of first trip through loop over offset/length
I/O vectors and always check for the contiguous storage sieve buffer buffer
being NULL.

    2 - Change the union containing the chunk and contiguous storage cache
information into a struct, allowing both to be used at the same time.


Platforms tested:
    FreeBSD 4.10 (sleipnir)
    h5committested