Fix crash in partcombine with empty music

[Reinhold Kainhofer]

Attached is a trivial patch for partcombine to avoid a crash when at least one 
of the two combined music expressions is empty:

I usually start with the general structure of a large score
and only then fill it with music. This means that partcombine
is fed two empty music expressions, so that evs1 and evs2 won't contain
data and (assoc "one" evs1) will be an empty list => cdr on
it will crash.
In this case, simply set the split list to an empty list and
don't crash...

Simple .ly example:
\version "2.11.35"
\partcombine { R1*15 } {}


Crash:
Warnung: keine Musik in der Partitur gefunden
Interpretation der 
Musik.../home/reinhold/lilypond/lilypond/out/share/lilypond/current/scm/part-combiner.scm:235:43:
 
In procedure cdr in expression (cdr (assoc "two" evs2)):
/home/reinhold/lilypond/lilypond/out/share/lilypond/current/scm/part-combiner.scm:235:43:
 
Wrong type (expecting pair): #f


Okay to commit?

Cheers,
Reinhold
-- 
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Reinhold Kainhofer, Vienna University of Technology, Austria
email: address@hidden, http://reinhold.kainhofer.com/
 * Financial and Actuarial Mathematics, TU Wien, http://www.fam.tuwien.ac.at/
 * K Desktop Environment, http://www.kde.org, KOrganizer maintainer
 * Chorvereinigung "Jung-Wien", http://www.jung-wien.at/

0001-Partcombine-Fix-crash-when-empty-music-is-passed.patch
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