string.c: fix integer overflow
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57797 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
string.c: fix integer overflow
only when SHARABLE_MIDDLE_SUBSTRING is enabled. incorpolate
https://github.com/mruby/mruby/commit/7db0786abdd243ba031e24683f
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57797 b2dd03c8-39d4-4d8f-98ff-823fe69b080e