Crash on navigate after delete in note input mode

Ubuntui 14.04, GIT commit: 00867de

1) note input mode
2) enter note
3) delete
4) left arrow

Result: crash

I added a call to Selection::cr() to the navigation code, but it is only safe to call if there is a single selection, and "Delete" clears the selection. I will change the code to return 0 rather than crash if there is no selection. I might suggest we consider having "Delete" within note input mode select something else (eg, when deleting a note, select the rest that is left in its place), but that's kind of a separate issue with other ramifications.