io.c (rb_io_syswrite): add RB_GC_GUARD
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44790 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
io.c (rb_io_syswrite): add RB_GC_GUARD
[Bug #9472][ruby-core:60407]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44790 b2dd03c8-39d4-4d8f-98ff-823fe69b080e