hdf5_der

[svn-r24999] Bugfix: HDFFV-8639 
h5diff segfaults with user's files

Solution:
Added a variable to indicate if real data are in the buffer and will call H5Dvlen_reclaim()
only when real data are in the buffer.

Tested: with the user provided data files and no more segmentaion fault.
Also h5committ tested.