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.


git-svn-id: https://svn.hdfgroup.uiuc.edu/hdf5/trunk@24999 dab4d1a6-ed17-0410-a064-d1ae371a2980