vchoi_fork

[svn-r17585] Purpose: Fix bug 1597

Description:
When copying a dataset  using a vlen inside a compound, the various dataset
copying callbacks would allocate a background buffer but would not use it when
converting from disk to memory, only memory to disk.  This caused an assertion
failure as compounds always need a background buffer.  These callbacks have
been modified to use the background buffer for both conversions.

Tested: jam, linew, smirom (h5committest)