Beam_collision_engraver sometimes fails on demisemiquavers or shorter notes

[Bertrand Bordage]

This isn't a regression. I suggest a medium or high prority.
Here is an example :

\version "2.15"
<<
  { a'32[ a' a' a'] }
  \\
  { c''[ d'' d'' d''] }
>>

Regards,
Bertrand