use 0 for reserved

use 0 for rb_data_type_t::reserved instead of NULL, since its type
may be changed in the future and possibly not a pointer type.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48662 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
common.mk: ext/json/parser/parser.c

* common.mk (ext/json/parser/parser.c): sometimes need for local
  development.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48663 b2dd03c8-39d4-4d8f-98ff-823fe69b080e