avcodec/atrac9dec: Check grad_range[1] more tightly

Alternatively the array could be made bigger but the extra values
would not be read without other changes.

Fixes: Out of array access
Fixes: 15658/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ATRAC9_fuzzer-5738260074070016

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 208225bd78)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/atrac9dec.c

@@ -121,7 +121,7 @@ static inline int parse_gradient(ATRAC9Context *s, ATRAC9BlockData *b,
     }
     b->grad_boundary = get_bits(gb, 4);
 
-    if (grad_range[0] >= grad_range[1] || grad_range[1] > 47)
+    if (grad_range[0] >= grad_range[1] || grad_range[1] > 31)
         return AVERROR_INVALIDDATA;
 
     if (grad_value[0] > 31 || grad_value[1] > 31)