string.c: fix for UTF-32
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40417 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
string.c: fix for UTF-32
contain NUL returns wrong result, use sizeof operator instead.
[ruby-dev:45975] [Feature #6752]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40417 b2dd03c8-39d4-4d8f-98ff-823fe69b080e