Fix fixed-point overflow in pitch downsampling

Reviewed by Mark Harris

celt/pitch.c

@@ -161,17 +161,26 @@ void pitch_downsample(celt_sig * OPUS_RESTRICT x[], opus_val16 * OPUS_RESTRICT x
       shift=0;
    if (C==2)
       shift++;
-#endif
    for (i=1;i<len>>1;i++)
-      x_lp[i] = SHR32(HALF32(HALF32(x[0][(2*i-1)]+x[0][(2*i+1)])+x[0][2*i]), shift);
-   x_lp[0] = SHR32(HALF32(HALF32(x[0][1])+x[0][0]), shift);
+      x_lp[i] = SHR32(x[0][(2*i-1)], shift+2) + SHR32(x[0][(2*i+1)], shift+2) + SHR32(x[0][2*i], shift+1);
+   x_lp[0] = SHR32(x[0][1], shift+2) + SHR32(x[0][0], shift+1);
    if (C==2)
    {
       for (i=1;i<len>>1;i++)
-         x_lp[i] += SHR32(HALF32(HALF32(x[1][(2*i-1)]+x[1][(2*i+1)])+x[1][2*i]), shift);
-      x_lp[0] += SHR32(HALF32(HALF32(x[1][1])+x[1][0]), shift);
+         x_lp[i] += SHR32(x[1][(2*i-1)], shift+2) + SHR32(x[1][(2*i+1)], shift+2) + SHR32(x[1][2*i], shift+1);
+      x_lp[0] += SHR32(x[1][1], shift+2) + SHR32(x[1][0], shift+1);
    }
-
+#else
+   for (i=1;i<len>>1;i++)
+      x_lp[i] = .25f*x[0][(2*i-1)] + .25f*x[0][(2*i+1)] + .5f*x[0][2*i];
+   x_lp[0] = .25f*x[0][1] + .5f*x[0][0];
+   if (C==2)
+   {
+      for (i=1;i<len>>1;i++)
+         x_lp[i] += .25f*x[1][(2*i-1)] + .25f*x[1][(2*i+1)] + .5f*x[1][2*i];
+      x_lp[0] += .25f*x[1][1] + .5f*x[1][0];
+   }
+#endif
    _celt_autocorr(x_lp, ac, NULL, 0,
                   4, len>>1, arch);