Avformat mux: fix a memory leak

(cherry picked from commit 1d3afcdb03b4dab9938ae311205e6364e34f6ec5)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/avformat/mux.c

@@ -161,6 +161,7 @@ void CloseMux( vlc_object_t *p_this )
     }
 
     avformat_free_context(p_sys->oc);
+    av_free(p_sys->io);
 
     free( p_sys->io_buffer );
     free( p_sys );