ffm: reject invalid codec_id and codec_type

A negative codec_id cannot be handled by the found_decoder API of
AVStream->info: if the codec_id is not recognized, found_decoder is set
to -codec_id, which has to be '<0' according to the API documentation.

This can cause NULL pointer dereferencing in try_decode_frame.

Also make sure the codec_type matches the expected one for codec_id.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
