Score corruption upon entering drum note in segment with no element in voice 1

Ubuntu 14.04, GIT commit: 949eefc

1) new score for drumset
2) empty measure, note entry
3) type B B B B to fill meaasure with bass drum notes in voice 2
4) cursor back to the second B (on beat two)
5) type A

Result: a snare drum note is entered onto beat 2, but the measure rest remains in place as well. The measure is now corrupt. The same is true if voice 1 original contained, say, a half note on beat 1. You will get a note added to beat 2 but the rest of the measure is not adjusted.

In regular (non drum) note entry, it is not possible to enter a note onto beat 2 unless there is already something there to replace - a note or rest. Trying to enter a note into voice in a context like this will simply enter the note onto beat 1. That seems the simple fix here as well.