avcodec/mpeg4videodec: Clear interlaced_dct for studio profile

Fixes: Out of array access
Fixes: 13090/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_fuzzer-5408668986638336

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Kieran Kunhya <kierank@obe.tv>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 1f686d023b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg4videodec.c

@@ -2968,6 +2968,7 @@ static int decode_studio_vop_header(Mpeg4DecContext *ctx, GetBitContext *gb)
         return 0;
 
     s->partitioned_frame = 0;
+    s->interlaced_dct = 0;
     s->decode_mb = mpeg4_decode_studio_mb;
 
     decode_smpte_tc(ctx, gb);