thread.c: error message

* thread.c (rb_thread_variable_get): fix error message.  getter
  doesn't modify thread variables.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40095 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
thread.c: avoid inadvertent symbol creation

* thread.c (rb_thread_aref): avoid inadvertent symbol creation.
  (rb_thread_variable_get): ditto.
  (rb_thread_key_p): ditto.
  (rb_thread_variable_p): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40096 b2dd03c8-39d4-4d8f-98ff-823fe69b080e