avformat/flvdec: RtmpSampleAccess no longer breaks stream detection

avformat/flvdec: RtmpSampleAccess no longer breaks stream detection

Since release 4.2, FFmpeg fails to detect the correct streams in an RTMP
stream that contains a |RtmpSampleAccess AMF object prior to the
onMetaData AMF object. In the debug log it would show "[flv] Unknown
type |RtmpSampleAccess".

This functionality broke in commit d7638d8dfc3c4ffd0dc18a64937a5a07ed67b354
as unknown metadata packets now result in an opaque data stream, and the
|RtmpSampleAccess packet was an "unknown" metadata packet type.

With this change the RTMP streams are correctly detected when there
is a |RtmpSampleAccess object prior to the onMetaData object.

Signed-off-by: Peter van der Spek <p.vanderspek@bluebillywig.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>