mxfdec: allow container_ul to override codec_ul if codec is A-law

Fixes some DNXHD files generated by AVID TM, where codec UL was set to A-law
meanwhile the real audio codec was PCM S16. According to SMPTE RP 224, A-law is
the default value for sound essence parameters therefore we should handle it
specially.

Signed-off-by: Marton Balint <cus@passwd.hu>
Reviewed-by: Tomas Härdin <tomas.hardin@codemill.se>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfdec.c

@@ -1536,7 +1536,8 @@ static int mxf_parse_structural_metadata(MXFContext *mxf)
             st->need_parsing = AVSTREAM_PARSE_HEADERS;
         } else if (st->codec->codec_type == AVMEDIA_TYPE_AUDIO) {
             container_ul = mxf_get_codec_ul(mxf_sound_essence_container_uls, essence_container_ul);
-            if (st->codec->codec_id == AV_CODEC_ID_NONE)
+            /* Only overwrite existing codec ID if it is unset or A-law, which is the default according to SMPTE RP 224. */
+            if (st->codec->codec_id == AV_CODEC_ID_NONE || (st->codec->codec_id == AV_CODEC_ID_PCM_ALAW && container_ul->id != AV_CODEC_ID_NONE))
                 st->codec->codec_id = container_ul->id;
             st->codec->channels = descriptor->channels;
             st->codec->bits_per_coded_sample = descriptor->bits_per_sample;