string.c: fix zero-length array
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56915 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
string.c: fix zero-length array
which is not a part of C90.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56915 b2dd03c8-39d4-4d8f-98ff-823fe69b080e