fix #278969: delete systems before measures on score destruction

As layout-only objects systems need to correctly update the state of
the related measures and spanners on its destruction. For this
reason it may introduce freed memory writing if score is destroyed
in an incorrect order.

libmscore/score.cpp

@@ -313,14 +313,15 @@ Score::~Score()
       foreach(MuseScoreView* v, viewer)
             v->removeScore();
       // deselectAll();
+      qDeleteAll(_systems); // systems are layout-only objects so we delete
+                            // them prior to measures.
       for (MeasureBase* m = _measures.first(); m;) {
             MeasureBase* nm = m->next();
             delete m;
             m = nm;
             }
       qDeleteAll(_parts);
       qDeleteAll(_staves);
-      qDeleteAll(_systems);
 //      qDeleteAll(_pages);         // TODO: check
       _masterScore = 0;
       }