Warn if muxing mpeg ps is attempted without a VBV buffer size.

Originally committed as revision 25385 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mpegenc.c

@@ -367,8 +367,10 @@ static int mpeg_mux_init(AVFormatContext *ctx)
             stream->id = mpv_id++;
             if (st->codec->rc_buffer_size)
                 stream->max_buffer_size = 6*1024 + st->codec->rc_buffer_size/8;
-            else
+            else{
+                av_log(ctx, AV_LOG_WARNING, "VBV buffer size not set, muxing may fail\n");
                 stream->max_buffer_size = 230*1024; //FIXME this is probably too small as default
+            }
 #if 0
                 /* see VCD standard, p. IV-7*/
                 stream->max_buffer_size = 46 * 1024;