Fixes some overflows in the fixed-point pitch code

The overflows reported were:
- Syy in find_best_pitch() in pitch.c
- xy, xx and yy in remove_doubling() in pitch.c

The fixes are:
- Adaptive scaling in pitch_downsample() which also improves quality
- Adding a missing downshift in find_best_pitch()
- More conservative yshift when calling find_best_pitch()

celt/mathops.h

@@ -128,6 +128,17 @@ static inline opus_val16 celt_maxabs16(opus_val16 *x, int len)
 }
 #endif
 
+#ifndef OVERRIDE_CELT_MAXABS32
+static inline opus_val32 celt_maxabs32(opus_val32 *x, int len)
+{
+   int i;
+   opus_val32 maxval = 0;
+   for (i=0;i<len;i++)
+      maxval = MAX32(maxval, ABS32(x[i]));
+   return maxval;
+}
+#endif
+
 /** Integer log in base2. Defined for zero, but not for negative numbers */
 static inline opus_int16 celt_zlog2(opus_val32 x)
 {