This fix makes sure new files save `wmWindow.global_areas` under a different
name, so old Blender versions don't recognize and 0-initialize it.
Since enabling global area writing (ef4aa42ea4), loading a file in old
Blender versions would cause `wmWindow.global_areas` to be read, because there
was already reading code for it and `ScrAreaMap` was in SDNA.
However the `ScrArea.global` of the global areas would be NULL, because it was
*not* in SDNA (`ScrGlobalAreaData` was excluded).
Now, issue is that the code assumes that areas in the global area-map have a
valid ScrArea.global pointer.
Think this was a mistake in rB5f6c45498c92. We should have cleared all this data
on reading, until the global area writing was enabled.
Differential Revision: https://developer.blender.org/D9442
Reviewed by: Brecht Van Lommel
390a0d5624