Blender Git Loki

Fix two issues related to 'partial' .blend files:

I) Filename was not put in temp Main generated to save selected data only,
this was breaking readcode when trying to open partial file, leading to missing
filename in final loaded Main data.

II) Read code would confuse partial .blend files with Undo ones, when they had no screen in them
(which happens to 99.999% of partial .blend files I guess).

Reported by @sybren, thanks.

Should be safe enough for 2.78 release.