Re: Issue 1336 in lilypond: skipTypesetting segfaults when set during a skipBars-induced MultiMeasureRest spanner

[lilypond]

Comment #5 on issue 1336 by percival.music.ca: skipTypesetting segfaults  
when set during a skipBars-induced MultiMeasureRest spanner
http://code.google.com/p/lilypond/issues/detail?id=1336

A bit more data: in the crash-case, grob-property.cc  
internal_get_property() calls internal_get_property_data () with  
minimum-X-extent, and receives the answer of
()
I'm guessing that at some other place, it tries to add up these values, and  
can't handle adding 0.0 and ().

I see two options:
1) make
Grob::internal_get_property_data (SCM sym) const
not return a () when looking up minimum-X-extent in this case.
2) robustify the addition such that it automatically treats () as a 0.

I'm guessing that #1 is the preferable option, but I have no real feeling  
for this stuff.  I won't be working on this for another week, so if anybody  
else wants to jump in now, go right ahead.