removing key signature causes crash

1. open new score with more than one staff (eg. piano)
2. drag key signature (e major) to some measure (not first key sig)
3. select the new key signature on one staff
4. delete it

This crashes b75d23e on Ubuntu 12.04

The problem is a null pointer (ks) in layout.cpp:923
wwMax = qMax(wwMax, ks->space().width());