Crash from embedded comment

[Trevor Bača]

Hi,

This causes Lily to crash:

### BEGIN ###

\version "2.21.80"

\new Score
<<

    \new GlobalContext { s1 }
    \new Staff { c'1 }

>>

\layout {
    \context {
        \name GlobalContext
        \type Engraver_group
        \consists Axis_group_engraver
        \override VerticalAxisGroup.default-staff-staff-spacing = #'(
            (basic-distance . 0)
            (minimum-distance . 12) % errant comment causes crash
            (padding . 0)
            (stretchability . 0)
        )
    }
    \context {
        \Score
        \accepts GlobalContext
    }
}

### END ###

GNU LilyPond 2.21.80
Processing `test.ly'
Parsing...
Interpreting music...
Preprocessing graphical objects...
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing
systems.../Applications/LilyPond.app/Contents/Resources/share/lilypond/current/scm/lily-library.scm:247:5:
In procedure ly:optimal-breaking in expression (process-procedure book
paper ...):
/Applications/LilyPond.app/Contents/Resources/share/lilypond/current/scm/lily-library.scm:247:5:
Wrong type (expecting pair): %

The input worked through the end of 2.19.

Presumably 2.21 changes something with the way LilyPond comments are parsed
from within Scheme blocks.

But this should error during parsing. And alert the user with a line number
from the input .ly file. The output shown above makes it look like there's
something wrong with lily-library.scm.

Trevor.

-- 
Trevor Bača
www.trevorbaca.com
soundcloud.com/trevorbaca