Assertion failure when using Melody_engraver and CueVoice

[Nate Whetsell]

Hi there,

Thank you for all your work on LilyPond!

It appears to be impossible to use a Melody_engraver in a Voice that also 
includes a CueVoice. Here’s a LilyPond example that illustrates the issue:

```
\version "2.22.0"
\new Voice \with {
  \consists "Melody_engraver"
  \override Stem.neutral-direction = #'()
} {
  <<
    { r }
    \new CueVoice { c'' }
  >>
}
```

LilyPond outputs:

GNU LilyPond 2.22.0
Processing `test.ly'
Parsing...
Interpreting music...
Preprocessing graphical objects...
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...Assertion failed: (d == 1 || d == -1), function at, file 
/private/tmp/lilypond-20210130-26352-lkbhbf/lilypond-2.22.0/build/../flower/include/drul-array.hh,
 line 35.

All the best,
Nate