Blender Git Loki

Fix T70315: Blender exit with code `-6` with message `Attempt to free NULL pointer`.

This is not actually fixing the real issue here, PackedFile structs are
never supposed to have a NULL pointer - and in that monster .blend file,
the pointer is not NULL, but the actual data chunk has been lost
somehow, so it gets NULL during read process.

Very unlikely we ever know how such corrupted .blend was created though
(there's probably a fair chance that this is not even due to a bug in
Blender, but rather a glitch in filesystem or something).

So for now, ensure at read time that we get a coherent state (i.e.
remove any read PackedFile that would have a NULL data field), and add a
few asserts in relevant code to check we never get NULL data pointer
here.