examples/demuxing_decoding: return error when no codec found

The open_codec_context function, when it fails to find a codec, now
return AVERROR(EINVAL) to signal an error.

Before it would return the stream index, which was always >= 0, and
continue as if a codec was found. This change make it fail faster,
instead of repeated failed tries to decode frames with no codec.
Signed-off-by: Even Wiik Thomassen <e.thomassen@sportradar.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/examples/demuxing_decoding.c

@@ -159,7 +159,7 @@ static int open_codec_context(int *stream_idx,
         if (!dec) {
             fprintf(stderr, "Failed to find %s codec\n",
                     av_get_media_type_string(type));
-            return ret;
+            return AVERROR(EINVAL);
         }
 
         /* Init the decoders, with or without reference counting */