allow NULL write_header() and write_trailer()

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@4675 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/utils.c

@@ -2253,9 +2253,11 @@ int av_write_header(AVFormatContext *s)
         }
     }
 
+    if(s->oformat->write_header){
         ret = s->oformat->write_header(s);
         if (ret < 0)
             return ret;
+    }
 
     /* init PTS generation */
     for(i=0;i<s->nb_streams;i++) {
@@ -2540,6 +2542,7 @@ int av_write_trailer(AVFormatContext *s)
             goto fail;
     }
 
+    if(s->oformat->write_trailer)
         ret = s->oformat->write_trailer(s);
 fail:
     if(ret == 0)