atrac9dec: relax gradient value requirements

Unlike the range, the gradient start value does not have to be lower
than the end value.
Does allow more files to be correctly decoded without errors.

Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>

libavcodec/atrac9dec.c

@@ -123,7 +123,7 @@ static inline int parse_gradient(ATRAC9Context *s, ATRAC9BlockData *b,
     if (grad_range[0] >= grad_range[1] || grad_range[1] > 47)
         return AVERROR_INVALIDDATA;
 
-    if (grad_value[0] >= grad_value[1] || grad_value[1] >= 32)
+    if (grad_value[0] > 31 || grad_value[1] > 31)
         return AVERROR_INVALIDDATA;
 
     if (b->grad_boundary > b->q_unit_cnt)