Resolves: tdf#100258 crash on disposing media player

SfxDockingWindow::dispose() will call SetFloatingMode false
its floating, which recreates the mpMediaWindow and so
the assert occurs about living children.

So toggle out of floating mode before deleting mpMediaWindow
to avoid that

Change-Id: I0b1cae149261fa43611b9f9a39465ce718e33484

avmedia/source/framework/mediaplayer.cxx

@@ -67,15 +67,18 @@ MediaFloater::~MediaFloater()
     disposeOnce();
 }
 
-
 void MediaFloater::dispose()
 {
+    if (IsFloatingMode())
+    {
+        Show(false, ShowFlags::NoFocusChange);
+        SetFloatingMode(false);
+    }
     delete mpMediaWindow;
     mpMediaWindow = nullptr;
     SfxDockingWindow::dispose();
 }
 
-
 void MediaFloater::Resize()
 {
     SfxDockingWindow::Resize();
@@ -84,7 +87,6 @@ void MediaFloater::Resize()
         mpMediaWindow->setPosSize( Rectangle( Point(), GetOutputSizePixel() ) );
 }
 
-
 void MediaFloater::ToggleFloatingMode()
 {
     ::avmedia::MediaItem aRestoreItem;