string.c: keep source code range
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45581 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
string.c: keep source code range
string even if code range of the destination string is unknown.
no reason to the former is affected by the latter.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45581 b2dd03c8-39d4-4d8f-98ff-823fe69b080e