avformat/flvdec: check FLVHeader PreviousTagSize0

refer to SPEC:
Annex E. The FLV File Format said:
E.3 TheFLVFileBody have a table:
Field            Type    Comment
PreviousTagSize0 UI32    Always 0

Reviewed-by: Bela Bodecs <bodecsb@vivanet.hu>
Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
