avformat mux: don't write trailer if writing header failed

(cherry picked from commit 62229a3b)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/avformat/mux.c

@@ -173,7 +173,7 @@ void CloseMux( vlc_object_t *p_this )
     sout_mux_sys_t *p_sys = p_mux->p_sys;
     unsigned int i;
 
-    if( !p_sys->b_write_header && av_write_trailer( p_sys->oc ) < 0 )
+    if( !p_sys->b_write_header && !p_sys->b_error && av_write_trailer( p_sys->oc ) < 0 )
     {
         msg_Err( p_mux, "could not write trailer" );
     }