Crash on paste

Windows 7, GIT commit: 6c5b016

This is not reproducible, but based on what I saw in debugger, it seems worth recording what happened, as it may be possible to deal with at least partially anyhow. Crash occurred here:

https://github.com/musescore/MuseScore/blob/master/mscore/scoreview.cpp…

system2 was 0. It is set a few lines earlier. When the crash occurred, I was in continuous view and had just pressed Ctrl+V to do a paste - a pretty simple paste of a couple measures of a single staff. I did not actually have mmrests turned on, although at some point I saw it had been turned on for me and corruption had been introduced. I can't say if this happened as a result of the crash, if it was the cause of the crash, or if it is unrelated.

Anyhow, the crash seems to occur while drawing the selection rectangle. I guess system2 should probably never be 0 based on how it is set, but we could at least detect that case and not go through with drawing the rectangle. Maybe that just puts off an inevitable problem later though.