Lyrics - Hyphens and Extenders bug

[Trent J]

Hello everyone,

I've found a bug in Windows Lilypond 2.9.10 and 2.9.13 (or 2.9.12 as it says 
when I runs).

When using hyphens and extenders with 'lyricsto' if there is a music line 
containing just hyphen's or extenders in the case of a large melisma then 
lilypond no longer prints the hyphens or extenders just the music line with 
nothing underneath. I've tried the example below in Lilypond 2.8.5 which 
works correctly the hyphens or extenders show (depending on how you change 
the lyrics) on all staves.

In versions 2.9.10 and 2.9.13 the hyphens and melismas disappear if there 
are no other lyrics on that line only the music is shown.

test.ly
==============

    <<
      \new Voice = "lala" \relative c'' {
        \time 3/4
        f4 g8
        \melisma
        f e f e2 e4 e2 e4 e2 e4
        e4 f e e4 e f e4 e f e4 g f
        f e f e2 e4 e2 e4 e2 e4
        e4 f e e4 e f e4 e f e4 g f
        f e f e2 e4 e2 e4 e2 e4
        e4 f e e4 e f e4 e f e4 g f
        f e f e2 e4 e2 e4 e2 e4
        e4 f e e4 e f e4 e f e4 g f
        f e f e2 e4 e2 e4 e2 e4
        e4 f e e4 e f e4 e f e4 g f
        e4 g\melismaEnd f
      }
      \new Lyrics \lyricsto "lala" {
        la di -- daah %changing to a hyphen -- or extender __ produces the 
bug
      }
    >>