Blender Git Loki

Two reported issues with undo #1667 and #1168

- press TAB without object active cleared undo stack
- typo (another!) in variable name, causing undo stack from wrong object
to be used (when have edited more objects)

Plus i found:
- rule for preserving editmode undo stacks, while using global undo, made
stricter now. Before it only checked name, now it checks for existance
of pointers extra.