avcodec/ffv1dec: Fix off by 1 error in quant_table_count check

Fixes: invalid_read.nut
Found-by: Paul B Mahol <onemda@gmail.com>

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 2d221d9e06)

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

libavcodec/ffv1dec.c

@@ -305,7 +305,7 @@ static int decode_slice_header(FFV1Context *f, FFV1Context *fs)
     for (i = 0; i < f->plane_count; i++) {
         PlaneContext * const p = &fs->plane[i];
         int idx = get_symbol(c, state, 0);
-        if (idx > (unsigned)f->quant_table_count) {
+        if (idx >= (unsigned)f->quant_table_count) {
             av_log(f->avctx, AV_LOG_ERROR, "quant_table_index out of range\n");
             return -1;
         }