Avoid add overflow in summation in AMR-WB decoder

codec look was doing a int32+= int16*int16 MAC which was overflowing
instead of saturating. Standard  3GPP #26.173 says it should saturate.

Bug: 112160454
Test: poc no longer crashes
Change-Id: I2979425be6b5b7b1dbe6790daf431fc86ab591c5