Stop playlist on quit, to fix some quitting crashes.

(cherry picked from commit 1d4f7191)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/dialogs_provider.cpp

@@ -93,6 +93,8 @@ DialogsProvider::~DialogsProvider()
 
 void DialogsProvider::quit()
 {
+    /* Stop the playlist */
+    playlist_Stop( THEPL );
     b_isDying = true;
     vlc_object_kill( p_intf->p_libvlc );
     QApplication::closeAllWindows();