Overextended extenders

[Michael Welsh Duggan]

When including a small descant lyric line, an extender at the end may
end up extending indefinitely.  I am working from git as of yesterday:

\version "2.13.51"

<<
  \new Lyrics = above \with {
    \override VerticalAxisGroup #'staff-affinity = #DOWN } s1
  \new Staff {
    \new Voice = "main" 
    \relative c'' {
      a1 
      <<
        \new Voice = "descanta" { \voiceOne c2 d( c1) }
        { \voiceTwo a2 a a a }
      >>
      a1
      % With the next note, the extender on the "oo" is too long.  If
      % the next note is removed, behavior is correct.
      a
    }
  }
  \new Lyrics \lyricsto "main" \lyricmode { 
    Ah, ah, ah, ah, ah, ah, ah. }
  \context Lyrics = above \lyricsto "descanta" \lyricmode { Oo -- oo __ }
>>

Output:

With the last "a" commented out:

-- 
Michael Welsh Duggan
(address@hidden)