instead of st_table.

* iseq.c (prepare_iseq_build): don't allocate ivar_cache_table
  until it has at least one element.

* iseq.c (compile_data_free): free ivar_cache_table only if it
  is allocated.

* compile.c (get_ivar_ic_value): allocate if the table is not
  allocated yet.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55002 b2dd03c8-39d4-4d8f-98ff-823fe69b080e