Redefined KVC_GPTR_NULL to (void*) 0.

Earlier, KVC_GPTR_NULL was defined as (kvc_gptr*) 0; however, the
splint tool seems unable to recognise that kvc_gptr* is an alias for
void*, and so issues false warnings. The redefinition has been done in
order to prevent these misleading errors from popping up.
Merge branch 'develop'