SDwritedata cannot change data of a compressed SDS after SDreaddata

Original issue: HDFFR-1356

When SDwritedata is called after SDreaddata, it cannot modify the data of a compressed data set. Note that this happens even in the case of writing the entire data set, which is supposed to work.

The problem can be reproduced using the files in this directory on jam: /mnt/hdf/qa/bugs/[HDFFR-1343](https://jira.hdfgroup.org/browse/HDFFR-1343)

SDScompressed.hdf is the original file containing one compressed SDS.
SDScompressed_wread.hdf is its copy to show whether the data was changed or not.

Keeping the SDendaccess/SDselect combo between SDreaddata and SDwritedata in the sample program modify_comp_SDS.c will show the values 887 being written in the SDS. Now, copy SDScompressed.hdf to SDScompressed_wread.hdf and try again with SDendaccess/SDselect commented out, 887 will not appear in the data. 887 is just a random value to show the changes.