Fix SystemStackError when calling a method in an unused refinement

Fixes [Bug #15720]

test/ruby/test_refinement.rb

@@ -2242,6 +2242,38 @@ class TestRefinement < Test::Unit::TestCase
     INPUT
   end
 
+  def test_call_method_in_unused_refinement
+    bug15720 = '[ruby-core:91916] [Bug #15720]'
+    assert_in_out_err([], <<-INPUT, ["ok"], [], bug15720)
+      module M1
+        refine Kernel do
+          def foo
+            'foo called!'
+          end
+        end
+      end
+
+      module M2
+        refine Kernel do
+          def bar
+            'bar called!'
+          end
+        end
+      end
+
+      using M1
+
+      foo
+
+      begin
+        bar
+      rescue NameError
+      end
+
+      puts "ok"
+    INPUT
+  end
+
   def test_super_from_refined_module
     a = EnvUtil.labeled_module("A") do
       def foo;"[A#{super}]";end

vm_insnhelper.c

@@ -2635,7 +2635,10 @@ vm_call_method_each_type(rb_execution_context_t *ec, rb_control_frame_t *cfp, st
 		    goto no_refinement_dispatch;
 		}
 	    }
-	    cc->me = ref_me;
+            if (cc->me->def->type != VM_METHOD_TYPE_REFINED ||
+                 cc->me->def != ref_me->def) {
+                 cc->me = ref_me;
+            }
 	    if (ref_me->def->type != VM_METHOD_TYPE_REFINED) {
 		return vm_call_method(ec, cfp, calling, ci, cc);
 	    }