struct.c: trivial optimization

* struct.c (rb_struct_set): get rid of calling rb_frame_this_func
  twice.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48740 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-12-08

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48741 b2dd03c8-39d4-4d8f-98ff-823fe69b080e