Avoid a malloc(0)

modules/demux/mp4/mp4.c

@@ -1688,10 +1688,13 @@ static int TrackCreateES( demux_t *p_demux, mp4_track_t *p_track,
                 if( p_avcC )
                 {
                     p_track->fmt.i_extra = p_avcC->data.p_avcC->i_avcC;
+                    if( p_track->fmt.i_extra > 0 )
+                    {
                         p_track->fmt.p_extra = malloc( p_avcC->data.p_avcC->i_avcC );
                         memcpy( p_track->fmt.p_extra, p_avcC->data.p_avcC->p_avcC,
                                 p_track->fmt.i_extra );
                     }
+                }
                 else
                 {
                     msg_Err( p_demux, "missing avcC" );