\crossStaff breaks, if stem-direction changes.

[Thomas Morley]

Hi,

a simple
\transpose c c'
(with changing stem-direction)
can break \crossStaff without printing a warning or error.

%%%%%%%%%%% example

\version "2.16.1"

\layout {
  \context {
    \PianoStaff
    \consists #Span_stem_engraver
  }
}

mI =
  \new PianoStaff <<
    \new Staff { d'8 }
    \new Staff { \crossStaff { g'8 } }
  >>

mII =
  \new PianoStaff <<
    \new Staff { \crossStaff { d'8 } }
    \new Staff { g'8 }
  >>

%% Good output
\transpose c c \mI

%% Bad output, no warning or error
\transpose c c' \mI

%% Good output, after manually fixing
\transpose c c' \mII

%%%%%%%%%%% end of example


-Harm