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: I451f353ba57487432904487d61107cf7bb818b8e
Merge topic 'if-before-delete2' into master

8812203b Conservative elimination of !null checks before 'delete'