Conservative elimination of !null checks before 'delete'

C++ guarantees that delete 0 and delete [] 0 do nothing.
Older buggy compilers that didn't honour this are no longer
supported by VTK.  Conservatively removed many "if (!ptr)"
checks before using delete in /Common.

Change-Id: Idf185d675e516e7fb5a8a745fd3dbf9720035325
Merge topic 'if-before-delete' into master

890a9760 Conservative elimination of !null checks before 'delete'