Fix 322957 and musescore#8592: fix issues discovered while moving gri…

…p handles around on trills

Two unrelated issues, neither really to do with trill lines specifically at all but both discovered at the same time:

a) if you drag the grip handle from any line into a 0-stave system (e.g. for a V-frame, as new scores have by default at the top of the score), it crashes
b) a bug in the layout engine where it reused cached information from a previous layout caused erratic behaviour when dragging grip handles around, though it could be triggered in other ways,

Backport of musescore#8600

libmscore/layout.cpp

@@ -2110,6 +2110,8 @@ System* Score::getNextSystem(LayoutContext& lc)
       if (!isVBox) {
             int nstaves = Score::nstaves();
             system->adjustStavesNumber(nstaves);
+            for (int i = 0; i < nstaves; ++i)
+                  system->staff(i)->setShow(score()->staff(i)->show());
             }
       return system;
       }

libmscore/line.cpp

@@ -403,7 +403,7 @@ Segment* LineSegment::findSegmentForGrip(Grip grip, QPointF pos) const
       System* sys = oldSeg->system();
       const QList<System*> foundSystems = score()->searchSystem(pos, sys, spacingFactor);
 
-      if (!foundSystems.empty() && !foundSystems.contains(sys))
+      if (!foundSystems.empty() && !foundSystems.contains(sys) && foundSystems[0]->staves()->size())
             sys = foundSystems[0];
 
       // Restrict searching segment to the correct staff