mosaic: use after free

(cherry picked from commit d0b6ecb24e52d943bbb2f0debdc4237b4b112aee)

modules/video_filter/mosaic.c

@@ -651,7 +651,7 @@ static subpicture_t *Filter( filter_t *p_filter, mtime_t date )
             p_filter->pf_sub_buffer_del( p_filter, p_spu );
             vlc_global_unlock( VLC_MOSAIC_MUTEX );
             vlc_mutex_unlock( &p_sys->lock );
-            return p_spu;
+            return NULL;
         }
 
         if( p_es->i_x >= 0 && p_es->i_y >= 0 )