avformat/matroskadec: Reject sipr flavor > 3

avformat/matroskadec: Reject sipr flavor > 3

Only flavors 0..3 seem to exist. E.g. rmdec.c treats any flavor > 3
as invalid data. Furthermore, we do not know how big the packets to
create ought to be given that for sipr these values are not read from
the bitstream, but from a table.

Furthermore, flavor is only used for sipr, so only check it for sipr;
rmdec.c does the same. (The old check for flavor being < 0 was
always wrong given that flavor is an int that is read via avio_rb16(),
so it has been removed completely.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>