deduplicate File::NULL string

"/dev/null" is a common sight for pre-1.9.3-compatible code
targeting *nix systems, so deduplicate it here, as well.

* file.c (Init_File): use fstring for File::NULL

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57996 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

file.c

@@ -6134,7 +6134,7 @@ Init_File(void)
     rb_define_const(rb_mFConst, "LOCK_NB", INT2FIX(LOCK_NB));
 
     /* Name of the null device */
-    rb_define_const(rb_mFConst, "NULL", rb_obj_freeze(rb_usascii_str_new2(null_device)));
+    rb_define_const(rb_mFConst, "NULL", rb_fstring_cstr(null_device));
 
     rb_define_method(rb_cFile, "path",  rb_file_path, 0);
     rb_define_method(rb_cFile, "to_path",  rb_file_path, 0);