flvdec: Check the avio_seek return value after reading a metadata packet

flvdec: Check the avio_seek return value after reading a metadata packet

If the metadata packet is corrupted, flv_read_metabody can accidentally
read past the start of the next packet. If the start of the next packet
had been flushed out of the IO buffer, we would be unable to seek to
the right position (on a nonseekable stream).

Prefer to clearly error out instead of silently trying to read from a
desynced stream which will only be interpreted as garbage.

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