[normative][txmg] Change clamp to normative range

This modification invalidates bitstreams that can cause
32-bit overflow when adding a rouding offset before a right-shift
operation.

BUG=aomedia:1512

Change-Id: I7ae6029edce3945fce9edcbdda4e490b7e893341

av1/common/av1_inv_txfm1d.c

diff --git a/av1/common/av1_inv_txfm1d.c b/av1/common/av1_inv_txfm1d.c
index cc88bd5..34d2c56 100644
--- a/av1/common/av1_inv_txfm1d.c
+++ b/av1/common/av1_inv_txfm1d.c
@@ -761,6 +761,7 @@
   output[1] = round_shift(x1, bit);
   output[2] = round_shift(x2, bit);
   output[3] = round_shift(x3, bit);
+  range_check_buf(6, input, output, 4, stage_range[6]);
 }
 
 void av1_iadst8_new(const int32_t *input, int32_t *output, int8_t cos_bit,