flvdec: Do not default to a video and audio stream

If no streams were indicated in the FLV header, do not automatically
allocate by default a video and an audio stream. Instead, in the case
that the header did not indicate the presence of any data, allocate no
stream until data actually arrives for one type.

Signed-off-by: Martin Storsjö <martin@martin.st>
rtmppkt: Add method to read an AMF string that is not prefixed by its type

Signed-off-by: Martin Storsjö <martin@martin.st>
rtmpproto: read metadata to set correct FLV header

In the presence of no metadata, do not set any stream flag in the FLV
header but let the demuxer handle the detection and creation of streams
as data arrives.

Signed-off-by: Martin Storsjö <martin@martin.st>