string.c: GC guard
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37143 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
string.c: GC guard
prevent String copies from GC. [ruby-core:47881] [Bug #7135]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37143 b2dd03c8-39d4-4d8f-98ff-823fe69b080e