view_builder: do_build_step: close build_step reader when done

Make sure to close the build_step reader before destroying it.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

db/view/view.cc

@@ -1763,7 +1763,10 @@ future<> view_builder::do_build_step() {
                 initialize_reader_at_current_token(_current_step->second);
             }
             if (_current_step->second.build_status.empty()) {
+                auto base = _current_step->second.base->schema();
+                auto reader = std::move(_current_step->second.reader);
                 _current_step = _base_to_build_step.erase(_current_step);
+                reader.close().get();
             } else {
                 ++_current_step;
             }