Beaming and slurs across staves

[Andrew Bernard]

I keep finding situations using beamed groups across staves where
adding a slur or a phrasing slur causes lilypond to fail with the
following error:


Processing `/home/andro/work/lilypond/fp/exp-slur-crash.ly'
Parsing...
Interpreting music...
Preprocessing graphical objects...
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
/home/andro/work/lilypond/fp/exp-slur-crash.ly:15:6: warning: no
viable initial configuration found: may not find good beam slope
c32
[\( c
lilypond: 
/home/gub/NewGub/gub/target/linux-64/src/lilypond-git.sv.gnu.org--lilypond.git-release-unstable/flower/include/interval.hh:227:
T Interval_t::center() const [with T = double]: Assertion `!is_empty
()' failed.

Refer to the MWE here for an example. This occurs in 2.19.41 at least.
I have not tried previous versions.

In other scenarios in my score even when the warning about not finding
a good beam slope is not emitted I still get the crash reliably in
this type of situation.

Is this a bug?

Andrew

== snip

\version "2.19.41"

treble = {
  \clef treble
  \time 1/4
  s4 s
}

bass =  {
  \clef bass
  \time 1/4

  %\once \override Beam.positions = #'(4 . 4)
  \stemUp
  c32[\( c
  \change Staff = treble
  \stemDown c' e' g' c'' c'
  \change Staff = bass
  \tuplet 5/4 { e c g, d32\rest e' }
  \change Staff = treble
  g'
  \change Staff = bass
  g
  \change Staff = treble
  a'']\)
}

\score {

  \new PianoStaff
  <<
    \new Staff = "treble" { \treble }

    \new Staff = "bass" { \bass }
  >>

  \layout {
  }
}

== snip