fix #39341: crash on tab entry

My recent fix for a cursor position bug, in #1453, removed what was effectively a recusion control check in Score::setPos().  Since I still don't see a better way to solve the original cursor position issue, I left that fix in place, but added a similar recursion check slightly further upstream.  It does appear this is the only place in the code where this would be an issue - none of the other calls to Score::setPlayPos() (which is where the call to setPos() happen anywhere that would be involved in the recursion chain.