reset bound if the size is 0.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66843 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
reset bound if the size is 0.
entries to calculate exact "bound" in ar_table, but if size == 0,
we can clear bound because there are no active entries.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66843 b2dd03c8-39d4-4d8f-98ff-823fe69b080e