avcodec/vorbisdec: Check vlc for floor0 dec vector offset

Fixes: out of array access
Fixes: 15649/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VORBIS_fuzzer-5729191309344768

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vorbisdec.c

@@ -1128,8 +1128,10 @@ static int vorbis_floor0_decode(vorbis_context *vc,
             ff_dlog(NULL, "floor0 dec: maximum depth: %d\n", codebook.maxdepth);
             /* read temp vector */
             vec_off = get_vlc2(&vc->gb, codebook.vlc.table,
-                               codebook.nb_bits, codebook.maxdepth)
-                      * codebook.dimensions;
+                               codebook.nb_bits, codebook.maxdepth);
+            if (vec_off < 0)
+                return AVERROR_INVALIDDATA;
+            vec_off *= codebook.dimensions;
             ff_dlog(NULL, "floor0 dec: vector offset: %d\n", vec_off);
             /* copy each vector component and add last to it */
             for (idx = 0; idx < codebook.dimensions; ++idx)