merges r30922 and r30924 from trunk into ruby_1_9_2.

--
* prevent temporary objects from GC, and should not use
  RSTRING_PTR() for function calls since it evaluates the argument
  a couple of times.
--
* thread.c (exec_recursive): prevent temporary objects from GC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_2@31765 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

variable.c

@@ -310,7 +310,8 @@ rb_class_name(VALUE klass)
 const char *
 rb_class2name(VALUE klass)
 {
-    return RSTRING_PTR(rb_class_name(klass));
+    VALUE name = rb_class_name(klass);
+    return RSTRING_PTR(name);
 }
 
 const char *