mosaic: fix potential deadlock (unlocking in the right order).

(cherry picked from commit 3e5af5bd)
Signed-off-by: Rémi Duraffort <ivoire@videolan.org>

modules/video_filter/mosaic.c

@@ -671,8 +671,8 @@ static subpicture_t *Filter( filter_t *p_filter, mtime_t date )
         {
             msg_Err( p_filter, "cannot allocate SPU region" );
             p_filter->pf_sub_buffer_del( p_filter, p_spu );
-            vlc_mutex_unlock( &p_sys->lock );
             vlc_mutex_unlock( p_sys->p_lock );
+            vlc_mutex_unlock( &p_sys->lock );
             return p_spu;
         }