Sequential_iterator: treat an initial invalid iterator specially

This corrects a problem introduced by commit fbca1f87 by using a more conservative condition that treats only the initial element specially.

My understanding of the actual problem is incomplete, but my resulting attempt to simplify Sequential_iterator has turned into a rewrite unsuitable for the current state of the LilyPond project—so here is the simple fix for the sake of stability.

Apparently, no regression tests used \lyricsto inside { } or << >> until now.

Closes #6036 (closed).