flvdec: Validate index entries added from metadata while reading

By validating the index entries while reading, we don't need to
seek at startup to validate the entries. If the error in the
index entries is not pointing to (our definition of) the start
of packets, and there is an index entry pointing at some of the
first packets after the metadata, the invalid index can be discarded
almost immediately.

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