[opus] Int overflow fix not present in tree.

[Timothy B. Terriberry]

Felicia Lim wrote:
> Hi Yves,
> 
> At that time, the issue wasn't reproducible in opus master, but there 
> had been some significant changes since the last tagged release that 
> made it hard to pinpoint a single patch to cherry pick to Chromium, 
> hence this local fix.

I'm not convinced that using a saturating add here is the correct thing 
to do, either (as opposed to, say, multiplying out by the coefficients 
individually).

Either way, there are exact analogs of this code in NSQ.c, as well as 
all of the SIMD optimizations (x86/NSQ_sse4_1.c and 
x86/NSQ_del_dec_sse4_1.c, arm/NSQ_del_dec_neon_intr.c, and 
mips/NSQ_del_dec_mipsr1.h). If it is a possible issue in master, the fix 
looks incomplete to me, at least.