Make CJBig2_Object::operator delete accept nullptr

Standard delete operator should do noting for nullptr.
Overloaded CJBig2_Object::operator delete de-referenced
provided pointer causing crashes after
https://codereview.chromium.org/1192743004/

TBR=thestig
BUG=508197

Review URL: https://codereview.chromium.org/1230763007 .