tracking down a crash

[Keith E OHara]

I was getting crashes while testing the page-layout changes on a big set of 
files, which went away when I set just one input file.  It will take a while 
before I narrow it down to a small example.

I can reproduce by taking 
<http://www.mutopiaproject.org/cgibin/piece-info.cgi?id=1793>
+ running convert-ly,
+ changing the signature of in trCueWhile in 'header.ily' so that the 
transposition argument is tested against ly:pitch?,
+ and then running `lilypond *.ly`.

A clean build of current master 2.19.0 segfaults on the second input file with 
the backtrace below.   I remember compiling the same input on 30 October's 
master with no problems.  Version 2.17.30 on Windows succeeds.

Invoking Lilypond individually for the .ly files also succeeds, so maybe this 
has something to do with compiling two input files, each setting volta repeats, 
in one session.  Next time I have time, I suppose I should pull the candidate 
2.18 stable branch and make sure there is no problem there.

===
(gdb) bt
#0  0x00000001 in ?? ()
#1  0x080a863a in Sequential_iterator::run_always (this=0xf0482d8) at 
sequential-iterator.cc:268
#2  0x080a863a in Sequential_iterator::run_always (this=0x10285b80) at 
sequential-iterator.cc:268
#3  0x083264ba in Music_wrapper_iterator::run_always (this=0x103a8090) at 
music-wrapper-iterator.cc:87
#4  0x083264ba in Music_wrapper_iterator::run_always (this=0x103853e8) at 
music-wrapper-iterator.cc:87
#5  0x0836eef1 in Simultaneous_music_iterator::ok (this=0x105461a8) at 
simultaneous-music-iterator.cc:127
#6  0x0815e2d8 in Global_context::run_iterator_on_me (this=0xfd77850, 
iter=0x105461a8) at global-context.cc:139
#7  0x0834510a in ly_interpret_music_expression (mus=0xac3ab898, 
ctx=0xb750a840) at global-context-scheme.cc:118