Philippe Gorley authored Nov 15, 2018 and Adrien Béraud committed Nov 19, 2018

Blindly adding both signals results in over/underflows which show up in
the signal as noise, avoid that by clamping the result between min and
max for s16 samples.

Change-Id: Ibbb42d87497c662b5ba9085d4c9efb6f1e45a183