[trunk] Note entry voice 2 crash for 5/4 measure

Steps to reproduce bug
1. Create a new score in 5/4 time
2. Select a full measure rest (if it is not already selected)
3. Press "N" to begin note entry
4. Select Voice 2
5. Press C on the keyboard to add a note to the staff

--> Crash

MuseScore version: r. 3738 nightly trunk

(Operating System: Windows 7)

first reported by fnbecker: http://musescore.org/en/node/8141