lavc/svq3: Fix regression decoding some files.

Fixes some SVQ3 encoded files which fail to decode correctly after 6d6faa2a.
These files exhibit lots of artifacts and logs show "Media key encryption is not implemented".
However they decode without artifacts before 6d6faa2a.
The attatched patch allows these files to successfully decode, but also reject media key files.

Tested on the files in #6094 and http://samples.mplayerhq.hu/V-codecs/SVQ3/Vertical400kbit.sorenson3.mov



Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>