examples/demuxing_decoding: return error when no codec found

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>