Crash pasting chords symbols from different staves

Ubuntu 14.04, GIT commit: 815dd63

See https://musescore.org/en/node/57221#comment-264216. I am pretty sure the problem is this:

- elements in symbol list come from multiple staves
- destination is last staff in score

On paste, I think we are trying to keep the same relative staff assignments, and aren't checking to see if there are enough staves. The specific crash comes when transposing the chord symbols, but I will want to see how we handle this for other symbols and try to be consistent.

Simplest would probably be to ignore symbols that would be pasted to a non-existent staff, which is basically how we handle copy and paste ordinary range selections I think. That is, if you select a range consisting of two staves, then paste to last staff, you get only the first staff of the select; the second staff of the source is simply ignored on the paste.