Crash in repitch mode entering note in voice >1 with no chord/rest at cursor

Ubuntu 14.04, GIT commit: 66cbc77

1) new score
2) note input mode
3) enable repitch mode
4) switch to voice 2
5) type "C"

Result: crash

There is no CR for voice 2 at the input position, so we are dereferencing a null pointer here:

https://github.com/musescore/MuseScore/blob/66cbc77363e585e48868ea0acbb…

A fix would presumably be to just return if _is.cr() is null. Better would probably be to do something like we are doing for rests here - skipping to the next valid input position.