fix(router): tree bug where loop index is not decremented. (#3460)

fixes https://github.com/gin-gonic/gin/issues/3459

routes_test.go

@@ -670,3 +670,22 @@ func TestRouteContextHoldsFullPath(t *testing.T) {
 	w := PerformRequest(router, http.MethodGet, "/not-found")
 	assert.Equal(t, http.StatusNotFound, w.Code)
 }
+
+func TestEngineHandleMethodNotAllowedCornerCase(t *testing.T) {
+	r := New()
+	r.HandleMethodNotAllowed = true
+
+	base := r.Group("base")
+	base.GET("/metrics", handlerTest1)
+
+	v1 := base.Group("v1")
+
+	v1.GET("/:id/devices", handlerTest1)
+	v1.GET("/user/:id/groups", handlerTest1)
+
+	v1.GET("/orgs/:id", handlerTest1)
+	v1.DELETE("/orgs/:id", handlerTest1)
+
+	w := PerformRequest(r, "GET", "/base/v1/user/groups")
+	assert.Equal(t, http.StatusNotFound, w.Code)
+}