Blender Git Loki

Fix for two proxy + undo related crashes:

* When making a proxy, the lib linked IPO driver was also changed to
point to the proxy object, and after undo this local proxy object
was replaced so the pointer became invalid. In fact it is not needed
at all to change this because the IPO code maps the pointer to the
local proxy object already.
* Undoing the make proxy operation would crash because the proxy_from
pointer in the library linked object would still point to the removed
object. Now it clears all these pointers before undo, because on each
undo memory file read they will be set again anyway.