Avoid undefined behavior after "music expected" error

This problem was introduced in 2.17.4 by an oversight in issue 2862.
Issue 3626: Remove unused iv in Side_position_interface::aligned_side

This was a leftover from issue 3026,
commit 2c0f56cc2c2b7e6d3a5f7792c006e55675c9e0d7
Issue3634: NR: Remove warning about requiring an explicit Voice for \cadenzaOn

Since issue 3140 (version 2.17.12) this has not been necessary.