crash in hdf_copy if src==NULL

```
patch:

=====
static NEOERR * _copy_nodes (HDF *dest, HDF *src)
{
  NEOERR *err = STATUS_OK;
  HDF *dt, *st;
  HDF_ATTR *attr_copy;

+  if (src == NULL)
+    return STATUS_OK;
  st = src->child;
  while (st != NULL)
  {
    err = _copy_attr(&attr_copy, st->attr);
=====
```

Original issue reported on code.google.com by `msink.p...@gmail.com` on 26 Sep 2012 at 11:48
