Removing staff causes crash if lines have nowhere to go

1. Create a multi-instrument score (such as the "Choir SATB.mscx" template)
2. Add a crescendo to the staff second from bottom
3. Create > Instruments
4. Remove both of the bottom two staves
5. Press OK

MuseScore crashes

Probably related to Lines do not move when new instrument added

(Tested using r. 2185 nightly, Windows XP)

First reported by dlanier: http://www.musescore.org/en/node/3063