Regression bug: Assertion error in 2.19.83

[Mats Bengtsson]

Hi,

The following example illustrates a bug in 2.19.83 that's a regression 
against 2.18. Compiling with 2.19.83 gives

GNU LilyPond 2.19.83
Processing `bug.ly'
Parsing...
Interpreting music...
Preprocessing graphical objects...lilypond: 
/home/gub/NewGub/gub/target/linux-64/src/lilypond-git.sv.gnu.org--lilypond.git-stable-test/flower/include/interval.hh:227: 
T Interval_t<T>::center() const [with T = double]: Assertion `!is_empty 
()' failed.
Abort (core dumped)

Uncommenting the first bar gives a file that can be processed and the 
second bar shows a workaround that works for the piece I'm typesetting. 
The example compiles without problems in 2.18.2, I don't have access to 
2.21, so I don't know if the bug remains there.

    /Mats