avcodec/mpeg4videodec: Clear partitioned frame in decode_studio_vop_header()

partitioned_frame is also set/cleared in decode_vop_header()

Fixes: out of array read
Fixes: 9789/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_fuzzer-5638681627983872

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 074187d599)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg4videodec.c

@@ -2967,6 +2967,7 @@ static int decode_studio_vop_header(Mpeg4DecContext *ctx, GetBitContext *gb)
     if (get_bits_left(gb) <= 32)
         return 0;
 
+    s->partitioned_frame = 0;
     s->decode_mb = mpeg4_decode_studio_mb;
 
     decode_smpte_tc(ctx, gb);