* hash.c: fix WB bug.
  (1) Hash's key also needs WB.
  (2) callback parameter *key and *value of st_update() is not a
  storage of st_table itself (only local variable). So that
  OBJ_WRITE() is not suitable, especially for `!exsinting'.
  OBJ_WRITTEN() is used instead of OBJ_WRITE().



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40979 b2dd03c8-39d4-4d8f-98ff-823fe69b080e