Saturate add to avoid int overflow.

Bug: 842528
Change-Id: Ic19806411dd784bf820a27a65abb1b6bb14517cc
Reviewed-on: https://chromium-review.googlesource.com/1072869Reviewed-by: Henrik Andreasson <henrika@chromium.org>
Commit-Queue: Felicia Lim <flim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561908}

third_party/opus/README.chromium

@@ -18,3 +18,5 @@ Local changes:
 * Apply https://github.com/xiph/opus/pull/73
 * Apply https://github.com/xiph/opus/pull/87
 * Make sure HB_gain is not NaN in an attempt to fix chromium:826914
+* Saturate add to avoid int overflow to fix chromium:842528. This should be
+  reverted when updating to v1.3

third_party/opus/src/silk/NSQ_del_dec.c

@@ -394,7 +394,7 @@ static OPUS_INLINE void silk_noise_shape_quantizer_del_dec(
         /* Long-term shaping */
         if( lag > 0 ) {
             /* Symmetric, packed FIR coefficients */
-            n_LTP_Q14 = silk_SMULWB( silk_ADD32( shp_lag_ptr[ 0 ], shp_lag_ptr[ -2 ] ), HarmShapeFIRPacked_Q14 );
+            n_LTP_Q14 = silk_SMULWB( silk_ADD_SAT32( shp_lag_ptr[ 0 ], shp_lag_ptr[ -2 ] ), HarmShapeFIRPacked_Q14 );
             n_LTP_Q14 = silk_SMLAWT( n_LTP_Q14, shp_lag_ptr[ -1 ],                      HarmShapeFIRPacked_Q14 );
             n_LTP_Q14 = silk_SUB_LSHIFT32( LTP_pred_Q14, n_LTP_Q14, 2 );            /* Q12 -> Q14 */
             shp_lag_ptr++;