avformat/mpegenc: improve field names in underflow error message

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mpegenc.c

@@ -906,7 +906,7 @@ static int remove_decoded_packets(AVFormatContext *ctx, int64_t scr){
             if(stream->buffer_index < pkt_desc->size ||
                stream->predecode_packet == stream->premux_packet){
                 av_log(ctx, AV_LOG_ERROR,
-                       "buffer underflow i=%d bufi=%d size=%d\n",
+                       "buffer underflow st=%d bufi=%d size=%d\n",
                        i, stream->buffer_index, pkt_desc->size);
                 break;
             }