Crash with simultaneous-duplicating music function and \once\offset

[Simon Albrecht]

Hello everybody,

this combination of a custom music function that combines the music 
simultaneously with itself and a \once\offset command causes LilyPond to 
crash (exit with return code 11):

%%%%%%%%%%%%
\version "2.19.82"
test =
#(define-music-function (parser location mus) (ly:music?)
  #{ << $mus $mus >> #})

\test { \once\offset length 1 Stem 4 }
%%%%%%%%%%%%

Best, Simon