Fix for invalid free error in hdfio.cxx
Fix for invalid free error in hdfio.cxx

Pbuf was sometimes being deleted when it was not allocated.
Modified the code to keep Pbuf either NULL or pointing at valid data
so we can check if it's allocated before we delete it.

pelahi/VELOCIraptor-STF,pelahi/VELOCIraptor-STF,pelahi/VELOCIraptor-STF