iSAC entropy coder: Avoid signed integer overflow

webrtc/modules/audio_coding/codecs/isac/fix/source/entropy_coding.c

Index: webrtc/modules/audio_coding/codecs/isac/fix/source/entropy_coding.c
diff --git a/webrtc/modules/audio_coding/codecs/isac/fix/source/entropy_coding.c b/webrtc/modules/audio_coding/codecs/isac/fix/source/entropy_coding.c
index 2379ba50661f2bac040ab2044ddc38a80be8b18a..c9ddf935801090e5959df3ee45bf38334011a4be 100644
--- a/webrtc/modules/audio_coding/codecs/isac/fix/source/entropy_coding.c
+++ b/webrtc/modules/audio_coding/codecs/isac/fix/source/entropy_coding.c
@@ -392,13 +392,13 @@ static void GenerateDitherQ7(int16_t *bufQ7,
       seed = WEBRTC_SPL_UMUL(seed, 196314165) + 907633515;
 
       /* fixed-point dither sample between -64 and 64 (Q7) */
-      dither1_Q7 = (int16_t)(((int32_t)seed + 16777216) >> 25);
+      dither1_Q7 = (int16_t)(((int32_t)(seed + 16777216)) >> 25);
 
       /* new random unsigned int32_t */
       seed = WEBRTC_SPL_UMUL(seed, 196314165) + 907633515;
 
       /* fixed-point dither sample between -64 and 64 */
-      dither2_Q7 = (int16_t)((seed + 16777216) >> 25);
+      dither2_Q7 = (int16_t)(((int32_t)(seed + 16777216)) >> 25);

[kwiberg-webrtc, 2016/02/25 11:57:00]

Here, I'm actually changing the behavior. The old code did an unsigned right
shift, while the new does a signed right shift (like in all the other places).
The new behavior is obviously the correct one; the old code gave results in the
range [0,127] instead of [-64,63].

[tlegrand-webrtc, 2016/02/26 10:21:40]

I think this changes should be fine.

 
       shft = (int16_t)(WEBRTC_SPL_RSHIFT_U32(seed, 25) & 15);
       if (shft < 5)
@@ -432,7 +432,7 @@ static void GenerateDitherQ7(int16_t *bufQ7,
       seed = WEBRTC_SPL_UMUL(seed, 196314165) + 907633515;
 
       /* fixed-point dither sample between -64 and 64 */
-      dither1_Q7 = (int16_t)(((int32_t)seed + 16777216) >> 25);
+      dither1_Q7 = (int16_t)(((int32_t)(seed + 16777216)) >> 25);
 
       /* dither sample is placed in either even or odd index */
       shft = (int16_t)(WEBRTC_SPL_RSHIFT_U32(seed, 25) & 1);     /* either 0 or 1 */