Strange programming error

[David Bobroff]

In recent CVS I'm getting this:

GNU LilyPond 2.5.21
Processing `/home/david/dean/ode/test.ly'
Parsing...
Interpreting music... [3]
Preprocessing graphical objects... 
Calculating line breaks... 
programming error: Stem has no extent in Slur_score_state
continuing, cross fingers
programming error: Stem has no extent in Slur_score_state
continuing, cross fingers
Calculating page breaks...
Layout output to `test.ps'...
Converting to `test.pdf'...

When processing this:

\version "2.5.21"

mus = \relative c' {
  \key d \major
  \clef F
  a2-( cis 
  a1-)
}

\score {
  \relative c' {
    \mus
  }
}

The output look ok, but it is weird to get this error from apparently
legitimate input.

-David