ffmpeg: force 128k default audio bitrate if nothing is specified and there is no specific default

This prevents breaking existing command lines in case the "ab" default is removed from libavcodec
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

ffmpeg.c

@@ -2604,6 +2604,11 @@ static int init_output_stream(OutputStream *ost, char *error, int error_len)
         if (!av_dict_get(ost->encoder_opts, "threads", NULL, 0))
             av_dict_set(&ost->encoder_opts, "threads", "auto", 0);
         av_dict_set(&ost->encoder_opts, "side_data_only_packets", "1", 0);
+        if (ost->enc->type == AVMEDIA_TYPE_AUDIO &&
+            !codec->defaults &&
+            !av_dict_get(ost->encoder_opts, "b", NULL, 0) &&
+            !av_dict_get(ost->encoder_opts, "ab", NULL, 0))
+            av_dict_set(&ost->encoder_opts, "b", "128000", 0);
 
         if ((ret = avcodec_open2(ost->enc_ctx, codec, &ost->encoder_opts)) < 0) {
             if (ret == AVERROR_EXPERIMENTAL)