Crash on score containing drum track with no drumset

OS: Windows , Arch.: x86_64, MuseScore version (32-bit): 2.2.9, revision: 3543170

As reported in https://musescore.org/en/node/273498

1) open attached score

Result: crash

There are two drumset staves with no drumset. This causes a crash in renderTremolo() as we try to deal with the tremolo. If you fix that, you get another in Note::ppitch() of the same ilk. I think the solution is to protect both passages with "if (ds)" - works for me, anyhow - but maybe there are other cases to consider.