hdf5_der

[svn-r19930] Fix memory leaks involving VL attributes in h5repack and h5diff.  The buffers in
copy_attr and diff_attr were not checked for the presence of a vlen before being
freed, and vlen storage was never reclaimed.  Added checks and calls to
H5D_vlen_reclaim().

Tested: Fedora (with valgrind)