Blender Git Loki

Fix T44894: Round two.

We cannot `direct_link_packedfile()` twice on a same address, because this tries to
map again pf->data address, which leads to nothing (NULL).

So now, since `ima->packedfile` and `ima->packedfiles` are mutually exclusive
in 'live' blender data anyway, we either read one or the other.

Tested from/to official 2.74 and current master, everything looks fine now.