Crash in Explode with multiple voices and only one staff selected

Ubuntu 14.04, GIT commit: 6bba0d4

1) new score for two flutes
2) flute 1, measure 1, voice 1: enter four quarter notes
3) flute 1, measure 1, voice 2: enter eight eighth notes
4) select fulte 1, measure 1
5) Tools / Explode

Result: crash

Code to calculate max number notes per chord wasn't checking that a chord existed in voice 1.