string.c: clear env self in symbol proc

* string.c (sym_to_proc), proc.c (rb_block_clear_env_self): clear
  caller's self which is useless, so that it can get collected.
  [Fixes GH-592]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45576 b2dd03c8-39d4-4d8f-98ff-823fe69b080e