Stop the avio input at a point where amf_parse_object can
continue parsing the end of the object seamlessly, when all
data is available.

If unsupported data is encountered within the keyframes object,
try seeking to the start of the keyframes object - if the seek
back was successful, the caller can continue parsing the rest
of the AMF data.
Signed-off-by: Martin Storsjö <martin@martin.st>