crash when changing instrument names' Layout Possition by too much

I can get 35b948a to crash, when changing the layoutPos of an instrument name foo far, getting an assertion failure:

Fatal: ASSERT failure in QList::operator[]: "index out of range", file C:/Qt/5.6/mingw49_32/include/QtCore/qlist.h, line 535 (:0, )

I guess in InstrumentName::setProperty() we need a bounds check

Setting it to 0 or 1 has the same effect for singlg staff instruments, 2 places it between the current and the next staff, 3 at the next staff, 4 crashes if that was the last staff. placing it above is not possible, negative values are not allowed.
Actually 0 and 1 differ for a 2-staff Instrument 1 is at top staff, 0 and 2 in the middle, 3 at lower staff.
It can't get set higher than 5, which should be enough for a 3-staff Instrument.