No validation for H5D_CHUNKED_REF_INDIRECT layout values

[jreadey]

The getChunkLocations in the SN node doesn't validate values found in the chunktable (e.g. that s3offset is a valid int).

[mattjala]

Possibly resolved in #420 - pending investigation

[mattjala]

#420 and the associated HDFGroup/hdf5-json#80 don't add any validation for these values. getChunkLocations still casts chunktable entries with a bare int and doesn't have any range/sanity checks, and h5json.dset_util.validateLayout doesn't check values from the chunktable.