PartcombineApart and overriding rests

[Clément]

> I'm not top posting.

Hello,

PartcombineApart refuses Rest's overrides.
In the first Voice, it does nothing. In the second Voice, it crashes.
When Rests are merged, no problem.
This bug does not affect MultiMeasureRests.

\version "2.17.3"

partI = \relative c'' {
  \partcombineApart
  \override Rest #'Y-offset = #4
  r2 c4 d r2 r
}

partII = \relative c' {
  \override Rest #'Y-offset = #-4
  r2 r r s
}

\new Staff { \partcombine \partI \partII }
\new Staff { \partI }
\new Staff { \partII }