avcodec/diracdec: Fix off by 1 error in quant check

Fixes: out of array read
Fixes: 1781/clusterfuzz-testcase-minimized-4617176877105152

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

libavcodec/diracdec.c

@@ -823,7 +823,7 @@ static int decode_hq_slice(DiracContext *s, DiracSlice *slice, uint8_t *tmp_buf)
     skip_bits_long(gb, 8*s->highquality.prefix_bytes);
     quant_idx = get_bits(gb, 8);
 
-    if (quant_idx > DIRAC_MAX_QUANT_INDEX) {
+    if (quant_idx > DIRAC_MAX_QUANT_INDEX - 1) {
         av_log(s->avctx, AV_LOG_ERROR, "Invalid quantization index - %i\n", quant_idx);
         return AVERROR_INVALIDDATA;
     }