Empty measures cause crash

1. Create a new score
2. Click on a rest in the first measure
3. Click on the "rest" button in the toolbar to remove the rest. This appears to create an empty measure
5. Now click on the beginning portion of the second measure (the empty space where the rest used to be)

MuseScore crashes.

(Tested using 0.9.4 stable and r.1809 development version)

First reported by Tony: http://musescore.org/en/node/1682