Fixes crash when running a unit test bundle while an app bundle runs.
Fixes crash when running a unit test bundle while an app bundle runs.

objc_allocateClassPair returns nil if the class pair already exists.
Calling objc_registerClassPair with a nil argument causes an EXC_BAD_ACCESS.

zjh171/PSTCollectionView,CorzFree/PSTCollectionView,zace-yuan/PSTCollectionView,jianwoo/PSTCollectionView,niuxinghua/PSTCollectionView,steipete/PSTCollectionView,Kevin775263419/PSTCollectionView,Anish-kumar-dev/PSTCollectionView,godLoveLucifer/PSTCollectionView,ba01ei/PSTImprovedCollectionView