tdf#131679 sw: follow-up: Unmark before SetSelection()

Backporting this to 6.4, it crashes in CppunitTest_desktop_lib because
some sidebar is loaded from SwView::AttrChangedNotify()/SelectShell()
and that ends up calling SwView::StateTabWin() about 40 stack frames
later and this calls SwFEShell::GetAnyCurRect() which gets the still
selected fly but its page frame is null.

So make sure shells don't see the deleted fly.

Change-Id: I6e994b26949d195aa9f326f4da193c69b0ee59c8
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/104815
Tested-by: Jenkins
Reviewed-by: Michael Stahl <michael.stahl@cib.de>

sw/source/core/layout/fly.cxx

@@ -412,6 +412,7 @@ void SwFlyFrame::FinitDrawObj()
                     if (SwFEShell *const pFEShell = dynamic_cast<SwFEShell*>(&rCurrentShell))
                     {   // tdf#131679 move any cursor out of fly
                         SwFlyFrame const*const pOldSelFly = ::GetFlyFromMarked(nullptr,  pFEShell);
+                        rCurrentShell.Imp()->GetDrawView()->UnmarkAll();
                         if (pOldSelFly)
                         {
                             SwPosition const pos(ResolveFlyAnchor(*pOldSelFly->GetFormat()));
@@ -421,7 +422,10 @@ void SwFlyFrame::FinitDrawObj()
                             // does, but that would access layout a bit much...
                         }
                     }
-                    rCurrentShell.Imp()->GetDrawView()->UnmarkAll();
+                    else
+                    {
+                        rCurrentShell.Imp()->GetDrawView()->UnmarkAll();
+                    }
                 }
             }
         }