Slurs not visible when staff above is hidden

GIT commit: 0f4127d

If you have two or more staves, when you hide the staff above one with slurs, the slur arcs are invisible. Steps to reproduce:

1. create a document with two staves
2. on the second staff, add two notes
3. select the notes and press s to slur
4. select Create->Instruments and uncheck Visible for the topmost instrument
5. verify the slur is not longer shown

Ubuntu 12.10, built myself in debug mode.