Description:
    Improve checks for NULL buffer from user during H5Dread/H5Dwrite calls.

Tested on:
    Linux/32 2.4.x (jam) w/gcc


git-svn-id: https://svn.hdfgroup.uiuc.edu/hdf5/trunk@24995 dab4d1a6-ed17-0410-a064-d1ae371a2980