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mcompand: fix bad clipping due to overflow of band summing.

When summing bands in mcompand I got some bad unnoticed clipping,
and the samples wrapped from positive to negative and viceversa.
With many bands sox_sample_t intergers get wrapped before being cast
to double.

--- a/ChangeLog

+++ b/ChangeLog

@@ -38,6 +38,7 @@

   o Improved pink noise frequency response in synth. (robs)

   o Extended syntax for specifying audio positions to several

     effects. (Ulrich Klauer)

+  o Fix integer overflow in mcompand. [3590093] (Guido Aulisi)

 Other new features:

--- a/src/mcompand.c

+++ b/src/mcompand.c

@@ -395,7 +395,7 @@

     (void)sox_mcompand_flow_1(effp, c,l,bbuf,abuf,len, (size_t)effp->out_signal.channels);

     for (i=0;i<len;++i)

     {

-      out = obuf[i] + abuf[i];

+      out = (double)obuf[i] + (double)abuf[i];

       SOX_SAMPLE_CLIP_COUNT(out, effp->clips);

       obuf[i] = out;

     }