avcodec/cbs: Only write extradata if there is something to write

It is e.g. legal for an ISOBMFF avcc to contain zero parameter sets.
In this case the annex B that we produce would be empty and therefore
useless. This happens e.g. with mov/frag_overlap.mp4 from the
FATE-suite.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/cbs.c

@@ -438,6 +438,10 @@ int ff_cbs_write_extradata(CodedBitstreamContext *ctx,
         return err;
 
     av_freep(&par->extradata);
+    par->extradata_size = 0;
+
+    if (!frag->data_size)
+        return 0;
 
     par->extradata = av_malloc(frag->data_size +
                                AV_INPUT_BUFFER_PADDING_SIZE);