Fix UBSan errors (left shift of negative value)

webrtc/modules/audio_coding/codecs/ilbc/get_lsp_poly.c

 /*
  *  Copyright (c) 2011 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 47 matching lines @@
   lspPtr+=2;
 
   for(i=2; i<=5; i++)
   {
     (*fPtr) = fPtr[-2];
 
     for(j=i; j>1; j--)
     {
       /* Compute f[j] = f[j] + tmp*f[j-1] + f[j-2]; */
       high = (int16_t)(fPtr[-1] >> 16);
-      low = (int16_t)((fPtr[-1] - ((int32_t)high << 16)) >> 1);
+      low = (int16_t)((fPtr[-1] & 0xffff) >> 1);

[tlegrand, 2016/05/17 12:24:32]

This is so much nicer!

[kwiberg-webrtc, 2016/05/17 12:32:29]

I've found the "x_low = x - x_high << 16" pattern in other places too. It
appears someone was in the habit of writing it that way...

Luckily, the compiler is smart enough to generate the same code regardless, so
this is just a source code improvement.

 
-      tmpW32 = ((high * *lspPtr) << 2) + (((low * *lspPtr) >> 15) << 2);
+      tmpW32 = 4 * high * *lspPtr + 4 * ((low * *lspPtr) >> 15);
 
       (*fPtr) += fPtr[-2];
       (*fPtr) -= tmpW32;
       fPtr--;
     }
-    *fPtr -= *lspPtr << 10;
+    *fPtr -= *lspPtr * (1 << 10);
 
     fPtr+=i;
     lspPtr+=2;
   }
   return;
 }