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)
c26ed138a6