Re: Fix crash when unknown grob name is passed to ly:engraver-make-grob (issue 5715053)

[plroskin]

Thanks for the ping, it's been a long time! Sadly, I haven't used
lilypond for years. I tried to update my patch, but I cannot spend too
much time on it.
I see that my change to engraver.cc is no longer needed, as there is a
check there that was missing back at the time. The change to
engraver-scheme.cc still applies, but I see that the comments here
indicate that it's incomplete and doesn't provide much value.
I have no recollection what I was doing with ly:engraver-make-grob, so I
googled to that symbol and found an implementation of ambitus at
http://lilypond.org/doc/v2.19/Documentation/snippets/contexts-and-engravers
I replaced "Ambitus" with "Ambituus" and I see the check added in
engraver.cc being triggered ("programming error: No grob definition
found for `Ambituus’") but the segfault happens not in
ly_engraver_make_grob but in Item::spanned_rank_interval.
So I think this patch is irrelevant and can be discarded.

https://codereview.appspot.com/5715053/