gc.c: avoid inadvertent pin-down

* gc.c (gc_info_decode): get rid of inadvertent dynamic symbol
  pin-down, and preserve encoding in error messages.  also should
  not use RSTRING_PTR macro on function calls.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45480 b2dd03c8-39d4-4d8f-98ff-823fe69b080e