BUG: Fix segfault if ~CleanUpObjectFactory called multiple times
BUG: Fix segfault if ~CleanUpObjectFactory called multiple times

This patch improves the robustness for library clean up, by setting a
global variable to null after it's deleted. This case may come about
when there are duplicate symbols for the object factory in the
executables symbol table from linking to ITK multiple times.

Change-Id: Iaaadb5703c4f7f01ab46b71820b815fcbcfacc22

Kitware/ITK,spinicist/ITK,hjmjohnson/ITK,BRAINSia/ITK,fbudin69500/ITK,BRAINSia/ITK,fbudin69500/ITK,stnava/ITK,PlutoniumHeart/ITK,LucasGandel/ITK,thewtex/ITK,fbudin69500/ITK,spinicist/ITK,blowekamp/ITK,BRAINSia/ITK,vfonov/ITK,thewtex/ITK,vfonov/ITK,LucasGandel/ITK,blowekamp/ITK,InsightSoftwareConsortium/ITK,spinicist/ITK,PlutoniumHeart/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,spinicist/ITK,PlutoniumHeart/ITK,blowekamp/ITK,richardbeare/ITK,vfonov/ITK,vfonov/ITK,blowekamp/ITK,thewtex/ITK,stnava/ITK,thewtex/ITK,fbudin69500/ITK,spinicist/ITK,fbudin69500/ITK,malaterre/ITK,fbudin69500/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,PlutoniumHeart/ITK,vfonov/ITK,stnava/ITK,BRAINSia/ITK,hjmjohnson/ITK,hjmjohnson/ITK,richardbeare/ITK,hjmjohnson/ITK,spinicist/ITK,PlutoniumHeart/ITK,malaterre/ITK,blowekamp/ITK,Kitware/ITK,hjmjohnson/ITK,Kitware/ITK,LucasGandel/ITK,stnava/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,malaterre/ITK,PlutoniumHeart/ITK,malaterre/ITK,stnava/ITK,BRAINSia/ITK,BRAINSia/ITK,hjmjohnson/ITK,malaterre/ITK,PlutoniumHeart/ITK,InsightSoftwareConsortium/ITK,stnava/ITK,spinicist/ITK,hjmjohnson/ITK,LucasGandel/ITK,vfonov/ITK,LucasGandel/ITK,malaterre/ITK,LucasGandel/ITK,Kitware/ITK,richardbeare/ITK,spinicist/ITK,richardbeare/ITK,fbudin69500/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,stnava/ITK,LucasGandel/ITK,malaterre/ITK,malaterre/ITK,PlutoniumHeart/ITK,thewtex/ITK,richardbeare/ITK,vfonov/ITK,blowekamp/ITK,thewtex/ITK,vfonov/ITK,LucasGandel/ITK,stnava/ITK,blowekamp/ITK,fbudin69500/ITK,thewtex/ITK,spinicist/ITK,Kitware/ITK,blowekamp/ITK,stnava/ITK